Why No More 9/11s? An interactive inquiry about why America hasn't been attacked again By Timothy Noah

Why No More 9/11s?

An interactive inquiry about why America hasn't been attacked again

By Timothy Noah

courtesy: http://www.slate.com/id/2213025

Posted Thursday, March 5, 2009, at 4:35 PM ET

The introduction to and eight parts contained in "Why No More 9/11s?" are reprinted here for those who prefer to read the entire series on paper. 

Amid the many uncertainties loosed by the al-Qaida attacks on Sept. 11, 2001, one forecast seemed beyond doubt: Islamist terrorists would strike the United States again—and soon. "Ninety days at the most," said counterterrorism expert Juval Aviv. On Oct. 5, 2001, an unnamed senior intelligence official told Congress, in a private briefing, that there was a "100 percent" chance of another terrorist attack should the U.S. invade Afghanistan, as it did two days later. "An attack is predictable now whether we retaliate against Afghanistan or not," reasoned House Speaker-to-be Nancy Pelosi, D-Calif. Sen. Richard Shelby, R-Ala., agreed: "You can just about bet on it."

When no second terrorist attack occurred in 2001, experts adjusted their time horizons. "If we get through the summer without some sort of attack, we'll be pretty fortunate," said George Vinson, a security adviser to then-California Gov. Gray Davis, in June 2002. In February 2003, Tom Ridge, the nation's first secretary of homeland defense, publicly estimated an 80 percent likelihood that terrorists would attack the United States within the next few days. In August 2003, the World Markets Research Center said it was "highly likely" that terrorists would attack the United States within the next 12 months. In June 2006, unnamed U.S. officials told CBS News they'd be surprised if the United States weren't hit by a terrorist attack by the end of that year. In December 2008, the Commission on the Prevention of WMD Proliferation and Terrorism said it was "more likely than not" that by the end of 2013, terrorists would attack somewhere in the world using a chemical, biological, or nuclear weapon. In a Feb. 4 interview with Politico, former Vice President Dick Cheney said there was "a high probability of such an attempt." He didn't say when.

It didn't happen—or, rather, it hasn't happened yet. Islamist terrorists struck Bali, Madrid, London, Mumbai, and many places in and around the Mideast, but they haven't struck the United States. Why not? The question is impossible to answer with certainty. But given that the "war on terrorism" was (for good or ill) the defining pursuit of George W. Bush's presidency, anyone seeking to understand the previous eight years of American political history must ask it. More urgently, our new president, Barack Obama, is surely pondering this question as he assesses the present risk of a terrorist attack on the United States and how best to address it.

I spent the Obama transition asking various terrorism experts why the dire predictions of a 9/11 sequel proved untrue and reviewing the literature on this question. The answers boiled down to eight prevailing theories whose implications range from fairly reassuring to deeply worrying.

I. The Terrorists-Are-Dumb Theory

The 9/11 attacks were heralded as the harbinger of a new era of foreign-based terrorism in the United States. What if they were really a fluke?

Al-Qaida's successful elimination of the Twin Towers, part of the Pentagon, four jetliners, and nearly 3,000 innocent lives makes the terror group seem, in hindsight, diabolically brilliant. But when you review how close the terrorists came to being exposed by U.S. intelligence, 9/11 doesn't look like an ingenious plan that succeeded because of shrewd planning. It looks like a stupid plan that succeeded through sheer dumb luck.

Consider:

• Conspirator Khalid Almihdhar, who was (at least theoretically) under U.S. surveillance for his suspected role in the bombing of the USS Cole, aroused suspicion at a San Diego flight-training school with his impatient request that he be taught how to fly a Boeing jet. (On 9/11, Almihdhar would help crash American Flight 77 into the Pentagon, killing 189 people.)

• Another conspirator, Nawaf Alhazmi, aroused suspicion when he boasted to a fellow gas-station employee that he would become famous. (Alhazmi would go down with Almihdhar on Flight 77. For more on what the Federal Bureau of Investigation knew about Almihdhar and Alhazmi prior to 9/11, click here.)

• By late July, former Central Intelligence Agency chief George Tenet told the 9/11 Commission, "the system was blinking red," and earlier that same month, FBI special agent Kenneth Williams sent a memo from the Phoenix office to Washington noting "an inordinate number of individuals of investigative interest" attending flight school in Arizona. (To read the memo, click here.)

• In early August, President Bush received a classified daily brief famously titled "Bin Laden Determined To Strike in U.S." (To read it, click here.)

• Later in August, the FBI's Minneapolis office interrogated Zacarias Moussaoui, who had aroused suspicion at a Minnesota flight school by asking about New York City flight patterns and whether a jetliner's cockpit doors could be opened while it was airborne. (To read a summary of what the Minneapolis office knew as of Aug. 19, click here.) Moussaoui's recklessness and volatility made his al-Qaida superiors reluctant to use him in the 9/11 attack; he was likely being held in reserve for a future al-Qaida attack, or possibly as a backup pilot for 9/11.

• Tenet, despite knowing "the system was blinking red," did nothing after he was briefed about Moussaoui on Aug. 23. (To read Tenet's "Terrorist Threat Review Update" for that day, click here.)

• In trying to obtain a warrant to examine Moussaoui's laptop, a field officer in the FBI's Minneapolis office told FBI headquarters that he was "trying to keep someone from taking a plane and crashing into the World Trade Center." The FBI did not obtain the evidence of Moussaoui's al-Qaida link deemed necessary to obtain that warrant until two days after 9/11.

Nearly eight years after the attacks, it remains physically sickening to review these for-want-of-a-nail details about what the U.S. government knew prior to 9/11. The various intelligence agencies' failures to pool their knowledge about the plot should surprise no one familiar with Washington's bureaucratic culture. But it's equally true that to count on so extreme a degree of government dysfunction, as al-Qaida effectively did, was foolhardy in the extreme. The terrorists took an unacceptably high risk that they'd get caught, and, just barely, they beat the odds. That they succeeded does not prove they were smart to try.

Nor is it clear that they were smart to succeed. In Afghanistan, the 9/11 attacks provoked a furious response from the United States military that destroyed al-Qaida's infrastructure of terrorist training camps and cave dwellings; unseated al-Qaida's protectors, the Taliban; and captured or killed two-thirds of al-Qaida's leaders—most notably, Khalid Sheikh Mohammed, 9/11's principal architect and now Guantanamo's best-known prisoner. According to Lawrence Wright, author of the Pulitzer Prize-winning The Looming Tower: Al-Qaida and the Road to 9/11, nearly 80 percent of al-Qaida's Afghanistan-based membership was killed in the U.S. invasion; intelligence estimates suggest al-Qaida's current membership may be as low as 200 or 300. At the very least, U.S. forces set back the al-Qaida hierarchy by several years. At most, the United States may have destroyed permanently al-Qaida's ability to operate as a centrally run enterprise, reducing its chairman, Osama Bin Laden, and its CEO, Ayman al-Zawahiri, to symbolic figureheads rather than hands-on leaders. Meanwhile, al-Qaida's goal of re-establishing the 1,000-year Islamic caliphate that sprawled across three continents until the early 20th century remains distant as ever. (The counterargument is that in striking back at Afghanistan and especially Iraq, the United States did itself incalculable damage by alienating much of the Muslim world while committing more blood and treasure than it could afford to lose. I elaborate this case in "The Time-Space Theory," at the opposite end of the worry spectrum.)

Despite some initial euphoria, 9/11 and its bloody aftermath appear to have worsened al-Qaida's standing not only among Muslims generally but even (Peter Bergen and Paul Cruikshank argued in a June 2008 New Republic article) among jihadis, who have observed that 9/11 led to the death of many more Muslims (especially in Afghanistan and Iraq) than anyone else. Many of these Muslims were killed directly by al-Qaida and other Islamist terrorists. In Overblown: How Politicians and the Terrorism Industry Inflate National Security Threats, John Mueller, a political scientist at Ohio State, relates how al-Qaida-linked coordinated suicide bombings in three Jordanian hotels in 2005 ended up killing a large group of Palestinians and Jordanians who were attending a wedding party. "It would be difficult," Mueller writes, "to imagine a target likely to be more stupid from the perspective of the terrorists." Members of al-Qaida in Iraq, an insurgent group affiliated with al-Qaida, alienated their fellow Sunnis by targeting civilians, imposing Taliban-style repression and provoking attacks from the country's Shiite majority. In response, Sunni militias banded together to form the Awakening, a U.S.-funded fighting force that, depending on whom you ask, deserves either some or most of the credit usually given to the U.S. troop surge for reducing the level of violence in Iraq since 2006.

Even some inside al-Qaida have come to view terrorism's costs as too high. In June 2008, Wright profiled a founding leader of al-Qaida known as Dr. Fadl. The author of two books used by al-Qaida to recruit members, Dr. Fadl more recently wrote from his Egyptian jail cell, "We are prohibited from committing aggression, even if the enemies of Islam do that." In Dr. Fadl's view, the 9/11 attacks were suicidal not only for the hijackers but for al-Qaida as a whole. Writing in 2007, Dr. Fadl argued that it was immoral to fight Muslim heads of state if they were believers and that it was unwise to do so if these leaders possessed sufficient power to prevail. Quoting the Prophet Mohammed, Dr. Fadl wrote: "Those who rebel against the Sultan shall die a pagan death." Dr. Fadl further argued that it was immoral under any circumstances to kill even non-Muslim civilians. These arguments stirred considerable debate in the Islamist world, and one can't rule out that Dr. Fadl produced them under threat of torture. But they were seconded by a Kuwaiti cleric named Sheikh Hamid al-Ali, who has been identified by the U.S. Treasury as an al-Qaida fundraiser. Al-Qaida's al-Zawahiri felt it necessary to write a lengthy rebuttal in which he claimed, preposterously, "no difference" between 9/11 and President Clinton's 1998 bombing of the Al-Shifa pharmaeutical plant in Sudan (believed, mistakenly, to be an al-Qaida chemical-weapons facility). The 1998 bombing killed one night watchman—a tragic blunder, but hardly comparable to the deliberate slaughter of 3,000 noncombatants.

However nonsensical, al-Zawahiri's views still prevail within al-Qaida. But does al-Qaida have the means for another U.S. attack? "Let's have some healthy skepticism … when we read about these terrorist masterminds who were poised to kill thousands of people and do incalculable damage," wrote cryptographer Bruce Schneier in a June 2007 column for Wired online titled "Portrait of the Modern Terrorist as an Idiot." Schneier believes that terrorism remains a real threat, but he also believes that the danger posed by the various alleged and would-be terrorists who've been caught inside the United States since 9/11, many of them lured into plots by undercover law enforcement, has been greatly exaggerated. (I'll have details about these sting operations in two later installments.) In The Looming Tower, Wright quotes Issam al-Turabi, a Bin Laden friend from his days in Sudan, thus: "I loved that man. … Unfortunately, his IQ was not that great."

Are terrorists dumb?

It may be that Bin Laden's family wealth and otherworldly dedication far outstrip his native intelligence. (Al-Zawahiri appears to be the brains of the operation.) But the real question isn't whether terrorists are smart per se but whether they are rational. "Acts of terrorism almost never appear to accomplish anything politically significant," prominent game theorist Thomas C. Schelling observed nearly two decades ago. Max Abrahms, a pre-doctoral fellow at Stanford's Center for International Security and Cooperation, reaffirmed that conclusion in a 2006 paper for International Security titled, "Why Terrorism Does Not Work." Abrahms researched 28 groups designated "foreign terrorist organizations" by the U.S. State Department since 2001, identifying among them a total of 42 objectives. The groups achieved those objectives only 7 percent of the time, Abrahms concluded, and the key variable for success was whether they targeted civilians. Groups that attacked civilian targets more often than military ones "systematically failed to achieve their policy objectives."

In a 2008 follow-up essay, "What Terrorists Really Want," Abrahms explained that terrorist groups are typically incapable of maintaining a consistent set of strategic goals, much less achieving them. Then why do they become terrorists? To "develop strong affective ties with fellow terrorists." It's fraternal bonds they want, not territory, nor influence, nor even, in most cases, to affirm religious beliefs. If a terrorist group's demands tend to sound improvised, that's because they are improvised; what really matters to its members—even its leaders—is that they are a band of brothers. Marc Sageman, a forensic psychiatrist and former Central Intelligence Agency case officer in Afghanistan, collected the biographies of 400 terrorists who'd targeted the United States. He found that fully 88 percent became terrorists not because they wanted to change the world but because they had "friendship/family bonds to the jihad." Among the 400, Sageman found only four who had "any hint of a [psychological] disorder," a lower incidence than in the general population. Think the Elks, only more lethal. Cut off from al-Qaida's top leadership, they are plenty dangerous, but not nearly as task-oriented as we imagine them to be.

II. The Near-Enemy Theory

There's no denying that al-Qaida wants to hurt the United States. The terror group repeatedly said so, through word and action, well before 9/11. In 2004, Osama Bin Laden boasted in a videotape that al-Qaida would "make America bleed profusely to the point of bankruptcy," much as he and his fellow jihadis had made the Soviet Union bleed in the 1980s by fighting Russian troops in Afghanistan. In truth, Bin Laden and the foreign fighters he led in Afghanistan played a peripheral role in chasing the Soviets out of Afghanistan—the real credit goes to the homegrown (and CIA-financed) mujahedeen—and the failed Soviet military intervention was only one of many factors that hastened the Soviet Union's dissolution. Even so, let us grant that al-Qaida means the United States serious harm and has managed to cause the United States considerable hardship. Is that an end in itself?

Of course not. Al-Qaida wants to bleed and bankrupt the United States not because it covets the territory that lies between Canada and Mexico but because it reviles U.S. influence in the Muslim world. We can argue about the extent to which that influence is wielded on behalf of progress (secularism, rule of law, democracy) or self-interest (cheap oil, geopolitical stability, development of markets for western goods and services). To al-Qaida, it scarcely matters. "Progress" and the advancement of U.S. interests are equally undesirable because they impede al-Qaida's sacred goal of resurrecting the 1,000-year caliphate.

In the previous essay ("The Terrorists-Are-Dumb Theory"), we saw Thomas Schelling, Marc Sageman, and Max Abrahms argue that terrorists think about strategy either very poorly or not at all. If that's the case, then al-Qaida attacks the United States mostly because it's there. But if terrorists are strategic thinkers, then al-Qaida's immediate goal would logically be to start building that caliphate by fostering the creation of jihadist regimes in the lands once conquered by the Prophet Mohammed and his successors. Following this logic, the need to attack the United States would vary according to how tightly the United States kept a lid on jihadis in the Middle East, South Asia, and North Africa. At the moment, the United States can't keep a very tight lid at all on Pakistan or Afghanistan, two places where al-Qaida has long maintained a presence. It would therefore make sense for al-Qaida to concentrate its resources there. Pakistan is a particular prize, because it has nuclear weapons; Bruce Riedel, formerly of the Central Intelligence Agency and now with the Brookings Institution, calls it "the most dangerous country in the world today." Two recent decisions by Pakistan authorities have caused particular alarm: an agreement with a key Taliban jihadi to impose Islamist law (Sharia) in the Swat valley, located within 100 miles of Pakistan's capital, Islamabad; and the release from house arrest of A.Q Khan, the scientist who sold nuclear secrets to North Korea, Iran, and Libya. In Afghanistan, the resurgent Taliban remains tightly knit with al-Qaida, and starting in 2005, it began adopting al-Qaida's tactic of suicide bombings. Both al-Qaida and the Taliban were implicated in the 2007 assassination of Pakistan's former Prime Minister Benazir Bhutto, and there's some evidence they also collaborated on a failed attempt to assassinate Afghanistan President Hamid Karzai in April 2008.

Jihadis speak of the "near enemy" (apostate regimes in and around the Middle East) and the "far enemy" (the United States and the West generally). The man credited with coining these terms, Mohammed Abd al-Salam Faraj, did so largely to emphasize that it was much more important to attack the near enemy, a principle he upheld by organizing the 1981 assassination of Egyptian President Anwar Sadat. (The Egyptian government affirmed the same principle in executing Faraj.) In 1993, a militant Egyptian group called al-Gama'a al-Islamiyya ("the Islamic Group"), which had extensive ties to al-Qaida, broke with the "near enemy" strategy and bombed the World Trade Center. In 1996, al-Qaida followed suit and formally turned its attention to the far enemy. But according to Fawaz A. Gerges, an international affairs professor at Sarah Lawrence and author of The Far Enemy: Why Jihad Went Global, other jihadist groups around the world never really bought into this shift in priorities. Even al-Gama'a al-Islamiyya had by late 1999 declared a cease-fire, a move that outraged its incarcerated spiritual leader, Omar Abdel-Rahman ("the blind sheikh") and caused the group to splinter. With the 9/11 attacks, Bin Laden hoped to rally jihadis outside al-Qaida's orbit to join the battle against the far enemy. Instead, he scared them off. Al-Qaida is today the only foreign terror group we know of with a declared interest in attacking the United States. That's why discussion—including this one—about whether the United States might experience another 9/11 typically focuses solely on al-Qaida. (I explain in "The Melting-Pot Theory" why the possibility of a domestic group causing another 9/11 is considered remote.) If al-Qaida isn't focused right now on attacking the United States, then there's no reason to believe anyone is.

I place the Near-Enemy Theory one stop further on the worry spectrum from the Terrorists-Are-Dumb Theory because even if al-Qaida is right now preoccupied with opportunities in its backyard, that doesn't necessarily keep it from devoting some resources to attacking the United States. The 9/11 attacks provoked the United States into invading Afghanistan and Iraq, and these invasions turned the Muslim world against the United States. In Pakistan, the percentage of the population holding favorable views about the United States (23 percent) fell by more than half (to 10 percent) between 1999 and 2002, according to the Pew Global Attitudes Project. Since then, it's crept up again to 19 percent. To whatever extent a new al-Qaida attack on the United States provoked further U.S. military action in the Muslim world, Pakistan's opinion of the United States would probably sour once again. Al-Qaida might consider that likelihood a valuable tool in its "near enemy" fight. Balanced against this, however, al-Qaida may have pondered where the U.S. troops would probably pour in: Pakistan and Afghanistan. That would be a likely setback in its "near enemy" fight.

III. The Melting-Pot Theory

In "The Terrorists-Are-Dumb Theory" I noted the terrible price al-Qaida paid in Afghanistan for 9/11. To repeat: Nearly 80 percent of its Afghanistan-based membership was killed in the U.S. invasion, according to journalist Lawrence Wright. Two-thirds of al-Qaida's leadership was captured or killed. The terror group's membership may now be down to as few as 200 or 300. Let's assume, as many believe, that this alone has made it very difficult for al-Qaida to stage a follow-up attack on the United States. Couldn't the job still be done by angry jihadis already living in the United States? Where are al-Qaida's sleeper cells?

Sleeping, apparently. Since 9/11, relatively few people have been prosecuted for conspiring with al-Qaida. In 2002, Brooklyn-born Jose Padilla was arrested in Chicago for allegedly plotting to set off a radioactive "dirty bomb," but five years later he would be tried (and convicted) on an entirely different charge concerning plans to commit terrorism abroad. In 2006, seven men from Liberty City in Miami were arrested for plotting to blow up the Sears Tower in Chicago with a Federal Bureau of Investigation informant posing as an al-Qaida terrorist. The FBI's own deputy director termed the conspiracy "more aspirational than operational," and the prosecution ended in a mistrial. In December 2008, the departing Bush administration put the total number of terrorists "and their supporters" arrested and convicted inside the United States since 9/11 at "more than two dozen," which would seem a weak effort but for the fact that no terror attacks occurred here during that time. In the absence of other evidence, we must conclude that inside the United States, homegrown, al-Qaida-inspired terrorist conspiracy-mongering seldom advances very far.

That record stands in stark contrast to that of the United Kingdom, which since 9/11 has incubated several very serious terrorism plots inspired or directed by al-Qaida. One of these reached fruition—the London Underground bombings of July 7, 2005, which killed 52 people and injured nearly 800 more. A follow-up attack two weeks later was thwarted only because the bombs failed to go off. Richard Reid, the foiled shoe bomber, boarded a Miami-bound plane in Paris but was a British citizen. The 2006 plot that begat airport bans on carrying liquids and gels also originated inside the United Kingdom and reportedly was in its final stages when halted by British authorities. In 2007, a suicide car bomber attacked the Glasgow airport but, fortunately, succeeded in killing only himself; around the same time British authorities found two unexploded car bombs in London's West End. Even when it isn't linked directly to terrorism, Muslim radicalism seems more prevalent—and certainly more visible—inside the United Kingdom, and in Western Europe generally, than it is inside the United States.

Why the difference? Economics may be one reason. American Muslims are better-educated and wealthier than the average American. In Europe, they are poorer and less well-educated than the rest of the population—in Germany, only about 10 percent of the Turkish population attends college. The United States has assimilated Muslims into its society more successfully than Western Europe—and over a longer period. Arabs began migrating to the United States in great numbers during the second half of the 19th century. Western Europe's Arab migration didn't start until after World War II, when many arrived as guest workers. In Germany and France, a great many Muslims live in housing projects segregated from the rest of the population. In the United States, Muslims are dispersed more widely. An exception would be Detroit, which has a large Muslim community but not an impoverished one.

Traditionally, the United States has been seen as less class-bound than Europe and more welcoming to immigrants. "There's something about our national myths," observes Jonathan Laurence, a political scientist at Boston College, "that give the impression of a possibility of political participation that European countries are still catching up to." Christopher Caldwell, author of a forthcoming book about Muslim immigration in Western Europe, notes also that the United States is unburdened by Europe's imperial legacy in the Middle East and South Asia. "Some of these relationships," he observes, "come pre-contextualized due to colonialism." Even so, Caldwell says Americans shouldn't get carried away with self-congratulation regarding their superior ability to bring Muslims into the mainstream. No Muslim ever won a seat in the House of Representatives until 2006, and none has ever won a Senate seat. (James Abourezk, who in the 1970s became the first Arab-American senator, is the son of Lebanese Christians.) At the same time, the seven medical doctors arrested in connection with 2007's bungled bombings in Glasgow and London clearly suffered no lack of educational opportunity or achievement.

The relative dearth of Islamist radicalism in the United States is at least as much a function of American demographics as it is of American exceptionalism. Muslims simply loom smaller in the U.S. population than they do in the populations of many Western European countries. Muslims account for roughly 3 percent of the population in the United Kingdom, 4 percent in Germany, and 9 percent in France. In the United States, they're closer to 1 percent and are spread over a much larger geographic area. As both immigrants and descendants of immigrants, Muslims are far outnumbered in the United States by Latinos. It's quite different in Western Europe. Muslims represent the largest single immigrant group in France, Germany, Belgium, the Netherlands (where they constitute a majority of all immigrants), and the United Kingdom (where they constitute a plurality of all immigrants).

Somewhere between one-quarter to one-half of U.S. Muslims are African-American. Historically, American-born black Muslims have felt little kinship with Arab and foreign-born Muslims, and while al-Qaida has sought to recruit black Muslims, "there's no sign" they've met with any success, according to Laurence. (Arabs make up less than half of 1 percent of the U.S. population, and a majority of them are Christian, mostly from Lebanon, Syria, and Egypt.) Among foreign-born Muslims in the United States, nearly one-quarter are Shiite—many of them refugees from the 1979 Iranian revolution—and therefore harbor little sympathy for al-Qaida's Sunni following. Europe's Muslim population, by contrast, is overwhelmingly Sunni, hailing typically in France from Algeria and Morocco; in Germany from Turkey; and in the United Kingdom from Pakistan and the subcontinent.

Whatever the reason, American Muslims appear far less inclined to support the global jihad than their European counterparts. In the United Kingdom, 81 percent of Muslims consider themselves Muslims first, British second. In the United States, only 47 percent consider themselves Muslim first. After 9/11, 58 percent of American Muslims said they approved of President Bush's response to the attacks. In a 2007 Pew survey, 51 percent said they were very concerned about the rise of Islamist extremism, a proportion the report termed "much greater than the concern expressed by Muslims in most of Western Europe, the Middle East, and elsewhere."

All right, then. American Muslims are disinclined to commit acts of terror inside the United States. Why don't American non-Muslims pick up the slack?

Actually, they do. In April 1995 Timothy McVeigh and Terry Nichols bombed a federal building in Oklahoma City, killing 168 people and injuring 500 more. In April 1996, Ted Kaczynski, the "Unabomber," was arrested for killing three people and wounding 22 others. In July 1996, a former Army explosives expert named Eric Rudolph set off a bomb at the Olympics in Atlanta, killing one person and injuring 11; later, he set off bombs at two abortion clinics and a nightclub frequented by gay men and women, killing a security guard* and injuring 12 others. In September and October 2001, somebody sent anthrax spores to media outlets and government offices, killing five people. The FBI believes it was an Army scientist named Bruce Ivins who killed himself as the investigation closed in on him. These are just the incidents everybody's heard of. The point is that domestic terrorism inside the United States is fairly routine. The FBI counted 24 terror incidents inside the United States between 2002 and 2005; all but one were committed by American citizens. (The exception was an airport attack that killed two people carried out by an Egyptian limousine driver who'd been living in the United States legally for 10 years.) Except for McVeigh and Nichols, however, these homegrown haters did not commit violence on a large scale. Fourteen years later, Oklahoma City looks more like a one-time event than a trend.

What places this theory one more bead along the worry chain is the unfortunate reality that it doesn't necessarily take very many people, Muslim or non-Muslim, to pull off a terrible attack. Only three people were prosecuted for the Oklahoma City bombing. Even 9/11 required only 19 suicide hijackers. The Melting-Pot Theory can't quiet entirely our anxiety about a hypothetical, statistically insignificant group of people who mean us harm.

[Update, March 4: Gallup and the Coexist Foundation have just released what appears to be the most extensive survey of American Muslims ever conducted. To read a summary click here. (The full report is available if you scroll to the bottom.)]

Correction, March 2, 2009: An earlier version of Part III. neglected to mention the death of the security guard, Robert Sanderson. (Return to the corrected sentence.)

IV. The Burden-Of-Success Theory

Ralph Ellison published his first novel, Invisible Man, in 1952. It won the National Book Award. The New York Times said Ellison had "mastered his art." In 1963, Ellison announced he would soon publish a second novel. The literary world held its breath. The book was delayed. Invisible Man, meanwhile, became recognized as perhaps the single greatest American novel of the postwar period. The years passed. "YOUR SILENCE PREVENTING WORK," Ellison telegrammed his future wife. In 1994 Ellison died, his second novel nowhere near completion.

Is Osama Bin Laden the Ralph Ellison of terrorism?

According to this theory, the 9/11 attacks were so stunning a success that they left al-Qaida's leadership struggling to conceive and carry out an even more fearsome and destructive plan against the United States. In his 2006 book The One Percent Doctrine, journalist Ron Suskind attributes to the U.S. intelligence community the suspicion that "Al Qaeda wouldn't want to act unless it could top the World Trade Center and the Pentagon with something even more devastating, creating an upward arc of rising and terrible expectation as to what, then, would follow." In a 2008 follow-up, The Way of the World, Suskind quotes Saad al-Faqih, a Saudi dissident believed by the U.S. Treasury to have ties to al-Qaida going back to the mid-1990s, predicting an attack "bigger than 9/11." The purpose of such escalation would be to incite a domestic uprising that would force the United States to retreat from the Muslim world and thereby "collapse the world order." The U.S. response to 9/11 in both Afghanistan and Iraq strongly suggests that precisely the opposite would happen, but never mind. "Terrorists compulsively drink deep from the well of their own propaganda," Bruce Hoffman, a terrorism expert at Georgetown's School of Foreign Service, wrote last year. "The movement doubtless continues to pin its hopes and faith on some new, spectacular terrorist attack that will catapult al-Qaida back into prominence."

An attack on this scale would very probably require a chemical, biological, or nuclear weapon. Al-Qaida is known to have pursued all three.

In 2001, the Wall Street Journal discovered a password-protected file titled "Yogurt" in a computer previously used by Ayman al-Zawahiri. "Yogurt" turned out to be the code name for a chemical and biological weapons project that al-Qaida had begun in 1999. "The destructive power of these weapons," al-Zawahiri had written excitedly (and inaccurately) in a memo, "is no less than that of nuclear weapons." Al-Zawahiri was particularly interested in developing an anthrax-based weapon and hired a microbiologist named Abdur Rauf to obtain the necessary spores and equipment. It's unclear precisely how far Rauf got. Al-Zawahiri also hired an Egyptian who went by the nom de guerre Abu Khabab to develop chemical weapons. This project developed to the point at which Khabab was able to test nerve gas on dogs and rabbits. (Today, Rauf is at large but under surveillance in Pakistan, which refuses to turn him over to the United States. Khabab was killed in July by an air strike from a CIA drone in the remote tribal region on the Pakistan-Afghanistan border where al-Qaida's top leaders relocated after the U.S. invasion of Afghanistan.)

There's scattered evidence these efforts are continuing. In July, a Pakistani neuroscientist named Aafia Siddiqui with suspected ties to al-Qaida was arrested in Afghanistan and extradited to New York on charges that she'd sought to kill U.S. troops. She is currently awaiting trial. When arrested, Siddiqui reportedly was found to possess documents about chemical, biological, and radiological weapons ("dirty bombs"). In late January, an al-Qaida affiliate in Algeria reportedly notified al-Qaida's leadership that it closed a facility to develop chemical or biological weapons after a fatal accident. The speculation was that the terrorists were trying to weaponize bubonic plague, though there is ample reason to be skeptical about that.

Al-Qaida has been seeking to acquire nuclear weapons since the early 1990s, when Osama Bin Laden got scammed to the tune of $1.5 million while trying to buy weapons-grade uranium. One month before 9/11, Bin Laden and al-Zawahiri met with Sultan Bashiruddin Mahmood, a key architect of Pakistan's nuclear program, known to possess nutty end-of-days beliefs about nukes and Islam. (Mahmood insisted, implausibly, that his sole purpose was to solicit funds for a polytechnic university he wished to establish in Kabul but that Bin Laden kept pestering him about building al-Qaida a nuclear bomb, which he refused to do. He's been under Pakistani surveillance ever since.) At the meeting, Bin Laden told Mahmood that he'd acquired nuclear material from Uzbekistan but that it wasn't of sufficient grade to make a nuclear weapon. News of this meeting helped fuel a flurry of panic at the Central Intelligence Agency in October 2001 over a report, later proved untrue, that al-Qaida had acquired a 10-megaton bomb stolen from Russia's nuclear arsenal. Al-Zawahiri has since boasted that al-Qaida possesses nuclear weapons, but that's highly doubtful. Pakistan's recent release from house arrest of nuclear scientist A.Q. Khan, who sold nuclear secrets to North Korea, Iran, and Libya, probably increased the risk of nuclear proliferation, but it's hard to know by how much.

Graham Allison, a Harvard political scientist of some renown, wrote in his 2004 book Nuclear Terrorism that "a nuclear terrorist attack on America in the decade ahead is more likely than not." When the paperback came out, he wrote in an afterword that "the likelihood, indeed inevitability, of a nuclear terrorist attack absent a major departure for current policy and practice" had increased over the previous year. In "World At Risk," a report about proliferation and terrorism released in December 2008, Allison and his fellow members of a congressional blue-ribbon panel pushed the deadline back to 2013, broadened the location to "somewhere in the world," and broadened the weapons category to include biological and chemical agents. Such predictions cause other terrorism experts to roll their eyes. John Mueller, a political scientist at Ohio State who believes the terrorism threat is overstated, twitted Allison for predicting as far back as 1995 that "acts of nuclear terrorism against American targets before this decade [i.e., the 1990s] is out."

In fact, the likelihood of nuclear terrorism isn't that great. Mueller points out that Russian "suitcase bombs," which figure prominently in discussions about "loose nukes," were all built before 1991 and ceased being operable after three years. Enriched uranium is extremely difficult to acquire; over the past decade, Mueller argues, there were only 10 known thefts. The material stolen weighed a combined 16 pounds, which was nowhere near the amount needed to build a bomb. Once the uranium is acquired, building the weapon is simple in theory (anti-nuclear activist Howard Morland published a famous 1979 article about this in the Progressive) but quite difficult in practice, which is why entire countries have had to work decades to acquire the bomb, only sometimes meeting with success. (Plutonium, another fissile material, is sufficiently dangerous and difficult to transport that nonproliferation experts seldom discuss it.)

Gathering material for a biological weapon may be somewhat easier, but actually fashioning that weapon would be harder, as witnessed by the fact that such weapons have scarcely ever been deployed, even by nations. On the rare occasions when they have been, they've failed to live up to their billing as weapons of mass destruction. "Perhaps the greatest disincentive to using biological weapons," John Parachini of the RAND Corporation testified before Congress in 2001, "is that terrorists can inflict (and have inflicted) many more fatalities and casualties with conventional explosives than with unconventional weapons." The same argument applies to chemical weapons. In theory, journalist Gregg Easterbrook has noted (citing a congressional report), under perfect conditions, one ton of sarin could kill up to 8,000 people. But it's "reasonably unlikely" that a terrorist group could acquire that much sarin, and perfect conditions mean no wind and no sun. Even light winds would reduce casualties to 800. You'd be better off detonating a conventional bomb in a city square.

On the other hand: Before 9/11 no self-respecting structural engineer would have predicted that you could reduce the World Trade Center towers and their inhabitants to dust by crashing two planes into them. The threat of an attack even more destructive than 9/11 is what risk analysts call "low-probability, high-risk." The likelihood is remote; the consequences would be devastating. Which probably makes the prospect as tempting to 9/11's murderous creators as finishing the Great American Second Novel was to Ralph Ellison.

V. The Flypaper Theory

The 9/11 attacks led to a U.S. invasion of Afghanistan, whose Taliban regime was sheltering al-Qaida. That made sense. Then it led to a U.S. invasion of Iraq. That made no sense. The Bush administration claimed that Iraq's Saddam Hussein had close ties to al-Qaida. This was based on:

a) allegations made by an American Enterprise Institute scholar named Laurie Mylroie, later discredited;

b) an al-Qaida captive's confession under threat of torture to Egyptian authorities, later retracted;

c) a false report from Czech intelligence about a Prague meeting between the lead 9/11 hijacker, Mohamed Atta, and an Iraqi intelligence agent;

d) Defense Secretary Donald Rumsfeld's zany complaint at a Sept. 12, 2001, White House meeting that "there aren't any good targets in Afghanistan, and there are lots of good targets in Iraq";

and

e) certain Oedipal preoccupations of President George W. Bush.

The purported terror link flatly contradicted the findings of intelligence agencies, and this became widely known to the public before the shooting started in Iraq. For the Bush administration, the absence of credible evidence linking Iraq and al-Qaida was deeply frustrating, especially after the other chief justification for the war—the presence of biological, chemical, and possibly nuclear weapons in Iraq—was disproved.

Then something wonderful happened. The al-Qaida link became true. After the U.S. invasion, Iraq was suddenly teeming with terrorists loyal to al-Qaida. Granted, this was terrible news for the nascent government in Iraq and for the American military, both of which came under violent attack as they tried to impose order. But it allowed President Bush to say, in effect: See? I told you the war in Iraq was part of the war on terror! Thus was born the Flypaper Theory.

The Flypaper Theory states that al-Qaida isn't attacking the United States because it's too busy attacking Americans in Iraq. Although sometimes mistaken for a strategy, this is, in fact, an after-the-fact justification. (If the Bush White House had expected al-Qaida to swarm into Iraq, it wouldn't have predicted prior to the invasion that American troops would be greeted "as liberators, not conquerors.") Lt. Gen. Ricardo Sanchez, then the top U.S. military commander in Iraq, may have been the first person to articulate the Flypaper Theory in a July 2003 interview with CNN's Wolf Blitzer:

This is what I would call a terrorist magnet, where America, being present here in Iraq, creates a target of opportunity, if you will. But this is exactly where we want to fight them. We want to fight them here. We prepared for them, and this will prevent the American people from having to go through their attacks back in the United States [italics mine].

President Bush rephrased this in a June 2005 speech to the nation:

Iraq is the latest battlefield in this war. Many terrorists who kill innocent men, women and children on the streets of Baghdad are followers of the same murderous ideology that took the lives of our citizens in New York and Washington and Pennsylvania. There is only one course of action against them: to defeat them abroad before they attack us at home [italics mine].

Responsible discussion of the Flypaper Theory requires a few caveats. For one thing, not all—or even most—of the insurgents battling U.S. troops in Iraq have been foreigners; in 2005, the Washington Post estimated foreigners represented 4 percent to 10 percent. Even al-Qaida in Iraq, the group to whom the Flypaper Theory seems most to apply, consists largely of Iraqis, and it's more a franchise of al-Qaida than a subsidiary. Another caveat is that the Central Intelligence Agency concluded as far back as 2005 that for Islamist extremists, Iraq was at least as much of a training ground as it was a flytrap. The number of anti-Western jihadis created by the Iraq war probably exceeds the number of anti-Western jihadis killed in the Iraq war.

For our purposes, though, the most significant caveat is that the Flypaper Theory has become at best a historical explanation, not a guide to current reality. There's considerably less fighting in Iraq today, and al-Qaida in Iraq has been on the ropes at least since 2007. The group's founder, Abu Musab al-Zarqawi, was killed in a June 2006 airstrike, and in May, his successor, Abu Ayyub al-Masri, suffered the humiliation of seeing the U.S. government bounty on his head be reduced from $5 million to $100,000. Iraq is no longer the jihadi diversion that it used to be and probably never was the jihadi diversion it was cracked up to be.

That's good news for Iraq but not such good news for Americans who worry about a follow-up to the 9/11 attack. To whatever extent al-Qaida and its affiliates were distracted by the war in Iraq, they aren't distracted now. What happened to all those trained jihadis? Are they redirecting their efforts to plot against the United States? We don't know. The Flypaper Theory earns its place in the worry spectrum not because of what it explains but because of the many imponderables it can't explain.

VI. The He-Kept-Us-Safe Theory

In his Jan. 15 farewell address, President George W. Bush said that after 9/11, "most Americans were able to return to life much as it had been before 9/11. But I never did." He continued:

Every morning, I received a briefing on the threats to our nation. I vowed to do everything in my power to keep us safe. … [T]here can be little debate about the results. America has gone more than seven years without another terrorist attack on our soil. This is a tribute to those who toil night and day to keep us safe—law enforcement officers, intelligence analysts, homeland security and diplomatic personnel, and the men and women of the United States Armed Forces.

A White House fact sheet specifies six terror plots "prevented in the United States" on Bush's watch:

an attempt to bomb fuel tanks at JFK airport,

a plot to blow up airliners bound for the East Coast,

a plan to destroy the tallest skyscraper in Los Angeles,

a plot by six al-Qaida-inspired individuals to kill soldiers at Fort Dix Army Base in New Jersey,

a plan to attack a Chicago-area shopping mall using grenades,

a plot to attack the Sears Tower in Chicago.

The Bush administration deserves at least some credit in each of these instances, but a few qualifications are in order. The most serious terror plot listed was the scheme to blow up airliners headed for the East Coast. That conspiracy, halted in its advanced stages, is why you aren't allowed to carry liquids and gels onto a plane. As noted in "The Melting-Pot Theory," it originated in the United Kingdom, which took the lead in the investigation. (The undercover agent who infiltrated the terror group was British.) We also learned in "The Melting-Pot Theory" that the plan to bring down the Sears Tower was termed by the Federal Bureau of Investigation's deputy director "more aspirational than operational" and that the prosecution ended in a mistrial.

The JFK plot was unrelated to al-Qaida and so technically infeasible that the New York Times, the airport's hometown newspaper, buried the story on Page A37. The attack on the Library Tower in Los Angeles was planned in October 2001 by 9/11's architect, Khalid Sheikh Mohammed, who recruited volunteers from South Asia to fly a commercial jetliner into the building. But Michael Scheuer, a veteran al-Qaida expert who was working at the Central Intelligence Agency in 2002, when the arrests were made, told the Voice of America that he never heard about them, and a U.S. government official told the Los Angeles Times that the plot never approached the operational stage. Moreover, as the story of United Flight 93 demonstrated, the tactic of flying passenger planes into buildings—which depended on passengers not conceiving of that possibility—didn't remain viable even through the morning of 9/11 ("Let's roll").

The Fort Dix plot was inspired by, but not directed by, al-Qaida. The five Muslim conspirators from New Jersey, convicted on conspiracy charges in December, watched jihadi videos. They were then foolish enough not only to make one of their own but to bring the tape to Circuit City for transfer to DVD. A teenage clerk tipped off the FBI, which infiltrated the group, sold them automatic weapons, and busted them. The attempted grenade attack on the CherryVale Mall in suburban Chicago was similarly inspired but not directed by al-Qaida. In this instance, the conspirators numbered only two, one of whom was an FBI informant. The other guy was arrested when an undercover FBI agent accepted his offer to trade two stereo speakers for four grenades and a gun. He is now serving a life sentence.

From a broader policy viewpoint, the Bush administration's most significant accomplishment, terrorism experts tend to agree, was the 2001 defeat of Afghanistan's Taliban regime and the destruction of Bin Laden's training camps. As noted in "The Terrorists-Are-Dumb Theory" and "The Melting Pot Theory," two-thirds of al-Qaida's leadership was captured or killed. Journalist Lawrence Wright estimates that nearly 80 percent of al-Qaida's Afghanistan-based membership was killed in the U.S. invasion, and intelligence estimates suggest al-Qaida's current membership may be as low as 200 or 300.

A 2007 National Intelligence Estimate stated that Bin Laden had "protected or regenerated key elements of its Homeland attack capability" by establishing a safe haven in Pakistan's tribal borderlands and through the appointment of operational lieutenants. On Feb. 25, Dennis C. Blair, the Obama administration's new director of national intelligence, told Congress that al-Qaida's leaders use this safe haven "as a base from which they can avoid capture, produce propaganda, communicate with operational cells abroad, and provide training and indoctrination to new terrorist operatives." But the Bush administration and Pakistan government responded to al-Qaida's improving capability by stepping up attacks on the tribal borderlands, and these continue under President Obama. According to unnamed Pakistani intelligence officials recently quoted in the New York Times, U.S. pilotless drone attacks are reducing the likelihood of an al-Qaida attack against the United States but increasing the likelihood that al-Qaida and the Taliban will destabilize Pakistan (see "The Near-Enemy Theory"), because the drones are killing many civilians along with the terrorists. The Bush administration struggled to keep these two considerations in balance. So will the Obama team.

Georgetown's Bruce Hoffman credits the National Counterterrorism Center, created in 2004, with breaking down much of the interagency resistance to sharing intelligence that proved fatal on 9/11. (See "The Terrorists-Are-Dumb Theory.") New procedures to screen commercial airline passengers and consolidate terrorist watch lists surely helped. Even the much-mocked Transportation Security Administration (nicknamed "Thousands Standing Around" in security-conscious Israel) has probably improved security, not because its methods are foolproof but because even a small increase in the risk of detection can make a big difference in a would-be terrorist's mental calculus. It's less clear that the doubling of border-patrol agents has had much effect, if only because policing U.S. borders remains a near-impossible task.

One Bush effort whose success is extremely difficult to gauge is the Treasury Department's tracking of terrorist funds. About $262 million in Taliban assets were blocked and then turned over to the new Afghan government after the U.S. invasion, and the Treasury's report on terrorist assets for 2007 (the most recent year for which data are available) lists $11 million in blocked al-Qaida assets (up from $8 million the previous year). According to the Central Intelligence Agency, before 9/11, al-Qaida had an annual budget of $30 million. Virtually none of this came from Osama Bin Laden's personal fortune, which was seized by the Saudis in 1994. As much as two-thirds of the al-Qaida budget may have been funneled directly to the Taliban as protection money. Richard Clarke, former White House counterterrorism chief, told Robert Windrem and Garrett Haake of MSNBC that the $30 million figure was "totally made up." Nobody even pretends to know how much money al-Qaida has now; most of it is probably in cash. The $11 million frozen by the U.S. government may be only a fraction of the amount that enthusiastic donors, exuberant about 9/11, kicked in after the attacks. On the other hand, getting cash-filled satchels to al-Qaida's top officers would surely have posed a steep challenge immediately after 9/11 and remains more difficult than it was before 9/11. On yet another hand, the 9/11 attacks cost only $500,000. Terrorism is a low-overhead business.

The departing Bush administration's claim that deposing Saddam Hussein helped prevent acts of terror in the United States has virtually no adherents, except to the extent that it drew some jihadis into Iraq. (See "The Flypaper Theory.") The Iraq war reduced U.S. standing in the Muslim world, especially when evidence surfaced that U.S. military officials had tortured and humiliated prisoners at the Abu Ghraib prison. The Bush White House fact sheet mentions not at all the Guantanamo internments and the Central Intelligence Agency's torture of terror suspects. That was probably a wise choice. But Vice President Dick Cheney defended these practices in exit interviews as he was leaving the White House, citing specifically the "wealth of information" provided by Khalid Sheikh Mohammed, who was reportedly water-boarded. "There was a period of time there, three or four years ago," Cheney said, "when about half of everything we knew about al Qaeda came from that one source." Capturing Sheikh Mohammed surely helped make America safe, but, as Slate's Dahlia Lithwick noted at the time, almost no one believes torture produces good information, and the number of people who believe it is legal "corresponds almost perfectly to the number of people who could be prosecuted for war crimes because it is not."

The noncontroversial parts of Bush's antiterrorism policies will continue under President Obama. The controversial parts probably won't. That troubles Cheney, who in February told Politico, "When we get people who are more concerned about reading the rights to an Al Qaeda terrorist than they are with protecting the United States against people who are absolutely committed to do anything they can to kill Americans, then I worry." If Cheney is right, then we're in greater danger under the Obama administration than we were under the Bush administration. If Cheney is wrong, then U.S. torture policies never provided much safety in the first place and may have made things worse by inflaming our enemies. Indeed, a recent two-part Washington Post piece suggested that abuse suffered by a Guantanamo detainee named Abdallah Saleh al-Ajmi transformed him from a relatively harmless Taliban foot soldier into a dedicated suicide bomber who, after his release, killed 13 Iraqi soldiers and wounded 42 others.

Either way, the government's ability to prevent another 9/11, while certainly greater than it was eight years ago, is surely incomplete. As with the Flypaper Theory, the He-Kept-Us Safe Theory offers cold comfort, because even if you accept every word of it as historically true, there are too many current and future contingencies that it can't address.

VII. The Electoral-Cycles Theory

President Bush liked to say that al-Qaida hated America because it was a democracy. That's true in the limited sense that Osama Bin Laden shows little interest in emulating that form of government. But if al-Qaida's purpose in attacking the United States is to provoke a massive domestic uprising to force a United States retreat from the Muslim world, as some believe (see "The Burden-of-Success Theory"), then Bin Laden ought to love that America is a democracy. Democracies, after all, are much more sensitive to shifts in public opinion than dictatorships. Indeed, elections may provide an especially handy occasion for al-Qaida to terrorize the public into effecting a radical change in government policy. Does al-Qaida time its actions accordingly?

Daniel Benjamin, former director for counterterrorism on the National Security Council in the Clinton White House (reportedly set to take the counterterrorism portfolio in the Obama State Department); Richard Clarke, the NSC's former national coordinator for security and counterterrorism in the Clinton and then the Bush White House; and Bruce Riedel, a former CIA officer, all believe that it does. Writing in Slate two weeks before the presidential election, Benjamin argued that elections are "seam moments, the points of inflection in history, and the terrorists want to demonstrate that they are central players in determining outcomes." Consider:

Less than one month before the 2000 presidential election, al-Qaida carried out a suicide bombing of the USS Cole, then docked in the Yemeni port of Aden.

Three days before Spain's March 2004 parliamentary elections, a local al-Qaida affiliate carried out train bombings in Madrid, creating a last-minute surge for the Socialist Party. The new government withdrew Spanish troops from Iraq.

Four days before 2004's U.S. presidential election, a videotape surfaced of Bin Laden telling Americans, "[Our] motivations are still there for what happened to be repeated." Democrat John Kerry has said he believes it cost him the election.

Three months before 2006's U.S. midterm congressional elections, British authorities shut down a planned coordinated attack by al-Qaida on jumbo jets flying to the United States. This conspiracy, which prompted the "liquids and gels" ban, was in a late stage. The GOP lost control of the House and Senate.

At the end of 2007, Pakistani extremists believed to be working with al-Qaida assassinated Benazir Bhutto, who had recently returned from exile to seek a third term as prime minister. Her husband became president as a result.

Did al-Qaida achieve its desired results? To believe that, you'd have to believe that al-Qaida preferred more dovish government in Spain but more hawkish government in the United States (except in Congress, which it preferred to be more dovish) and that for some reason it preferred Asif Ali Zardari to his wife. That wouldn't make much sense. It's possible al-Qaida harbored incorrect notions about how these various events would play out. Al-Qaida is typically credited with preferring hawkish foes to dovish ones because that throws the "clash of civilizations" into greater relief, but who really knows? Does al-Qaida favor certain candidates or parties? Benjamin concedes there's little evidence to support that notion. "If al-Qaeda attacks occur when they are most convenient for the attackers," argues Benjamin H. Friedman, a terrorism expert at the Cato Institute, they will be randomly distributed throughout the year, meaning that a certain [proportion], which will head toward one-twelfth as years go by, will fall in the month before elections. Citing a few attacks that occured around election time is evidence of nothing.

Another difficulty is that the big one, 9/11, occurred nearly one year after a major U.S. election. The 1993 bombing of the World Trade Center, which bore links to al-Qaida, occurred three months after a major U.S. election. Al-Qaida's 1998 embassy bombings in Tanzania and Kenya occurred nearly two years after a presidential election and three months before a midterm congressional election in which the biggest issue was the fallout from President Clinton's affair with a White House intern. Election-cycle theorists finesse most of this by arguing that the danger period lasts through the first year of a new presidency. That's because a chief executive still learning the ropes is likelier to blunder either in defending against an attack or responding to it. Both proved particularly true of President Bush.

Might President Obama be similarly vulnerable? His secretary of state thought so during the primary campaign. "I don't think it was by accident that Al-Qaeda decided to test [Britain's] new prime minister," candidate Hillary Clinton said in January 2008, referring to an al-Qaida-linked car bombing at Glasgow airport mere days after Gordon Brown moved into 10 Downing Street. "They watch our elections as closely as we do." During the general campaign, Vice President Biden made a similar point. "Mark my words," he said. "It will not be six months before the world tests Barack Obama." Biden didn't say the test would come from Osama Bin Laden, but that's certainly possible. I give this theory the penultimate bead because if Biden is right, then we have entered a period of maximum danger.

VIII. The Time-Space Theory

The RAND Corp. is headquartered in a blindingly white temple of reason a few blocks from the Pacific Ocean in Santa Monica, Calif. It was here—or rather, next door, in the boxy international-style offices it inhabited for half a century before moving four years ago into a new $100 million structure—that America's Cold War nuclear strategy of "mutual assured destruction" was dreamed up. Also, the Internet. Created by the Air Force in 1948, the nonprofit RAND would "invent a whole new language in [its] quest for rationality," Slate's Fred Kaplan wrote in his 1983 book The Wizards of Armageddon.

RAND is the cradle of rational-choice theory, a rigorously utilitarian mode of thought with applications to virtually every field of social science. Under rational-choice theory, belief systems, historical circumstances, cultural influences, and other nonrational filigree must be removed from consideration in calculating the dynamics of human behavior. There exists only the rational and orderly pursuit of self-interest. It is the religion that governs RAND. "You can leave your backpack in my office," RAND senior economist Darius Lakdawalla told me as we headed for a conference room. "There's no theft at RAND." I asked whether "externalities" were permitted anywhere inside the building. He chuckled politely.

Lakdawalla and RAND economist Claude Berrebi are co-authors of "How Does Terrorism Risk Vary Across Space and Time?" a 2007 paper. (To download a copy, click here and pay $30. To download an earlier draft of the same paper free of charge, click here.) An underlying assumption of Berrebi and Lakdawalla's analysis is that—contrary to arguments put forward by game theorist Thomas C. Schelling; Max Abrahms of Stanford's Center for International Security and Cooperation; and Marc Sageman, a forensic psychiatrist and former CIA case officer—terrorists pursue concrete goals in a rational way. On the small-bore tactical level, Berrebi says, terrorists are very rational. It's entirely possible, Lakdawalla explains, to pursue an irrational goal in a rational manner. Berrebi cites terrorists' tendency to use suicide bombers only when no other alternatives are available. In general, terrorists prefer poorly protected targets to well-protected ones. "When stuff becomes harder to hit," Lakdawalla says, "terrorist groups, like anyone else, tend to look for easier opportunities." Don't worry about getting killed at the airport, where security measures have proliferated. Worry about getting killed at the shopping mall, where your only protection is a bored security guard.

When Schelling, Abrams, and Sageman argue that terrorists are irrational, what they mean is that terror groups seldom realize their big-picture strategic goals. But Berrebi says you can't pronounce terrorists irrational until you know what they really want. "We don't know what are the real goals of each organization," he says. Any given terror organization is likely to have many competing and perhaps even contradictory goals. Given these groups' inherently secret nature, outsiders aren't likely to learn which of these goals is given priority.

One goal inherent in the 9/11 attacks was to do harm to the United States. In "The Terrorists-Are-Dumb Theory" and "The Melting-Pot Theory," we reviewed the considerable harm that the furious U.S. response to 9/11 caused al-Qaida. But that response harmed the United States, too. Nearly 5,000 U.S. troops have died in Iraq and Afghanistan, and more than 15,000 have come home wounded. More than 90,000 Iraqi civilians have been killed and perhaps as many as 10,000 Afghan civilians; in Afghanistan, where fighting has intensified, more than 2,000 civilians died just in the past year. "In Muslim nations, the wars in Afghanistan and particularly Iraq have driven negative ratings [of the United States] nearly off the charts," the Pew Global Attitudes Project reported in December. Gallup polls conducted between 2006 and 2008 found approval ratings for the U.S. government at 15 percent in the Middle East, 23 percent in Europe, and 34 percent in Asia. To be sure, civilian casualties have harmed al-Qaida's standing, too, as I noted in "The Terrorists-Are-Dumb Theory." But to whatever extent al-Qaida hoped to reduce the United States' standing in the world, and especially in the Middle East: Mission accomplished. The Pew survey found most countries anticipated an improvement under President Obama, who already has ordered that Guantanamo be shuttered and that the Bush administration's creative interpretations of the Geneva Convention be revoked. But with the Obama administration escalating troop levels in Afghanistan and a growing sense that U.S. forces will remain in Iraq for years to come, the United States won't be loved by the Muslim world anytime soon.

Rational-choice theory is most at home with economics, and here the costs are more straightforward. In March 2008, the Nobel Prize-winning economist Joseph Stiglitz, and Linda Bilmes of Harvard's Kennedy School of Government, put the Iraq war's cost at $3 trillion. In October 2008, the Congressional Research Service calculated, more conservatively, an additional $107 billion for the Afghanistan war and another $28 billion for enhanced homeland security since 9/11. According to CRS, for every soldier the United States deploys in Iraq or Afghanistan, the taxpayer spends $390,000. Let me put that another way. Sending a single soldier to Iraq or Afghanistan costs the United States nearly as much as the estimated $500,000 it cost al-Qaida to conduct the entire 9/11 operation. Not a bad return on Bin Laden's investment, Berrebi says. President Bush left office with a budget deficit of nearly $500 billion, and that's before most of the deficit spending that most economists think will be required to avoid another Great Depression even begins. "We are facing the recession starting from a worsened standpoint," Berrebi says. Al-Qaida may not be the only reason, but it certainly helped.

Berrebi and Lakdawalla's "How Does Terrorism Risk Vary Across Space and Time?" focuses on Israel's experience, but it can be applied to the question of why al-Qaida hasn't attacked the United States since 9/11. The "space" referred to in the title is the particular site chosen for a terrorist attack. Proximity to the terrorist headquarters and easy access to an international border are, Berrebi and Lakdawalla write, hugely important in deciding whether to carry out a terrorist attack. "When distance to a terrorist home base doubles," they calculate, "the frequency of attacks falls by around 30 percent." Areas near international borders "are more than twice as likely to be hit" as areas far from international borders. Following this logic, Israel is a veritable paradise for Islamist terrorists. It's located right in the Middle East, and, east to west, it's only 85 miles wide. The United States, by contrast, is a jihadi's worst nightmare: halfway across the world and 3,000 miles wide. The two countries' comparative experience with terrorism reflects these realities.

The "time" referred to in the title is the interval between attacks, and it's a lot less comforting to people who live in the United States. To simplify things as we spoke in a RAND conference room, Berrebi drew a rudimentary graph on a whiteboard. The vertical axis represented the risk of attack in a regional capital. The horizontal axis represented the passage of time. It looked like this:

The risk of attack increases sharply before an attack occurs (duh), then falls, then levels off and eventually starts to rise again. I mentioned to Berrebi and Lakdawalla that my girlfriend had travelled to New Delhi a couple of weeks after the Mumbai terror attack and that I'd worried about her. Guess I shouldn't have worried! Berrebi shook his head and tapped on the line just to the right of the first peak. The risk immediately after an attack, he said, is still very high, because it takes the authorities some time to figure out what's happening and to beef up security—and because the terrorists' planned mission may not be completed. Gradually, security measures are put in place. Then gradually, these security measures slacken, creating new opportunities for attack. In Jerusalem, Berrebi and Lakdawalla found that after a terror attack the risk of a follow-up attack begins to increase after only two incident-free months. "This suggests," they conclude, "that long periods of quiet actually indicate elevated risk for sensitive areas." Berrebi and Lakdawalla are restating the familiar war-movie cliché in which two soldiers stand guard over a peaceful nighttime landscape. "It's quiet," says one. "Yeah," says the other. "Too quiet." Then the enemy emits a battle cry and the fighting begins.

In Jerusalem, it's two months from peak to trough. In the United States, it's X months, where X has an unknown value greater than 44.* If the United States suffers another major terror attack, Berrebi and Lakdawalla will be able to calculate the value of X. They think they'll probably get that second data point. They just don't know when.