Given the scale of the damage caused to the United States, the 9/11 attacks neither required much money to execute, nor did they take a large number of plotters. Terrorism is a cheap form of warfare--the first World Trade Center attack in 1993, for instance, only cost a few thousand dollars. This is particularly the case when you have a cadre of young men willing to engage in suicidal terrorism. According to court documents entered in the trial of Al-Qaeda's Zacarias Moussaoui, the entire 9/11 operation cost a little over $200,000, a trivial sum considering the damage it inflicted on the United States. Furthermore, no amount of money will buy you 19 young men willing to commit suicide in a terrorist operation. The pilots who flew the hijacked planes into two of the world's most famous buildings saw what they were doing as an act of worship.

The success of the attacks relied above all on the faith of the hijackers that they were doing God's will. Al-Qaeda's strength lies not in its material resources, which are relatively trivial, but in the nature of its beliefs. Unfortunately, since 9/11 we have seen the Al-Qaeda ideological virus spread widely, partly as a result of the war in Iraq. The spread of that virus can be gauged by an epidemic of suicide terrorism around the world that first spiked in 2003, and has reached unprecedented proportions in the past year from Afghanistan to Iraq to the United Kingdom to Egypt.

The 9/11 Commission report makes clear that one of the reasons the attacks succeeded was that Osama bin Laden intervened to make two key decisions that ensured their success. The first was to appoint Mohamed Atta to be the lead hijacker. Atta would carry out his responsibilities with grim efficiency. The second decision was to rein in the operational commander of 9/11, Khaled Sheik Mohammed (KSM), who planned for ten planes to crash into targets both on the East Coast and simultaneously in Asia. Bin Laden reasoned that so many attacks would be hard to synchronize and might lead to the failure of the entire plan.

During the trial of Zacarias Moussaoui, interrogation summaries obtained from KSM--he had been captured in Pakistan in late 2003--were entered into evidence. In those summaries KSM outlined the dictatorial powers that Bin Laden exercised over his organization:
""If the Shura (consultative) council at Al-Qaeda, the highest authority in the organization, had a majority of 98 percent on a resolution and it is opposed by Bin Laden, he has the right to cancel the resolution.""

Bin Laden's total dominance of Al-Qaeda meant it was hostage to his strategic vision, and that became a problem for the organization because Bin Laden's cult-like control over Al-Qaeda is not matched by any depth of strategic insight. Bin Laden's analysis of American foreign policy is based on the U.S. pull out from Lebanon in 1983, after the Marine barracks attack that killed 241 American servicemen, and the U.S. withdrawal from Somalia in 1993, after 18 U.S. soldiers were killed in Mogadishu. From these American retreats, Bin Laden concluded the United States to be a "paper tiger" capable of only withstanding a few strikes before it would fall, taking down with it also its client regimes in the Middle East. This would turn out to be a disastrously naive view of the American response to 9/11, which was in fact to destroy the Taliban regime and decimate Al-Qaeda. In short, Bin Laden came to believe his own propaganda--always a dangerous mistake--and 9/11 turned out to be something of a kamikaze operation for Al-Qaeda.

The extent of the self-inflicted wound that Al-Qaeda suffered as a result of 9/11 can be seen in an extraordinary letter written in June 2002 by one Abd-Al-Halim Adl to "Mukhtar", an alias for KSM. The letter, which was recovered by the U.S. military in Afghanistan, was written at a time that KSM was planning additional attacks around the world. The letter bemoans the disasters that befell Al-Qaeda after the fall of the Taliban:

"We must completely halt all external actions until we sit down and consider the disaster we caused. The East Asia, Europe, America, Horn of Africa, Yemen, Gulf, and Morocco [terrorist] groups have fallen . . . My beloved brother, stop all foreign actions, stop sending people to captivity, stop devising new operations, regardless of whether orders come or do not come from Abu Abdallah (Bin Laden). Our adherents have lost confidence in us and in our ability to manage the action, and they wonder, what has befallen us."

This letter shows that a year after 9/11, Al-Qaeda insiders believed that the organization was severely damaged. However, four years later, Al-Qaeda is showing signs of renewed vigor. The London bombings of July 2005 that killed 56 people were Al-Qaeda operations. Two of the suicide attackers recorded their "wills" with As Sahab, Al-Qaeda's video production arm in Pakistan. Meanwhile, judging by the blizzard of tapes they have released this year, Al-Qaeda's leaders don't seem too bothered by the war on terror. Bin Laden has released five audiotapes while his deputy, Ayman al-Zawahiri, has appeared in an unprecedented number of videotapes. Further, after the death of Al-Qaeda's leader in Iraq, Abu Musab al-Zarqawi, his successor, Abu Hamza al-Mujaher, quickly pledged allegiance to Bin Laden. Mujaher has been a member of Zawahiri's jihad group since 1982 and will likely align himself closely with Al-Qaeda's leaders.

It is not simply that Al-Qaeda has managed to regroup from its base on the Afghan-Pakistan border and can, therefore, initiate another attack on the United States. The situation is more complex: many of the underlying problems and factors that led to the attacks in the first place continue to fester. None of these underlying causes alone are sufficient to explain 9/11, but taken together they form the toxic brew that precipitated the attacks. And they will continue to create the conditions from which another 9/11 might spring:

Decline and stagnation in the Middle East, and feelings of humiliation in the Muslim world. The historian Bernard Lewis is the principal exponent of the idea that the Muslim world is in a crisis largely attributable to centuries of long decline from prominence embodied in the fate of the once powerful Ottoman Empire and its ignominious post-World War I carve up by the British and French. Lewis also explains that in the mid-twentieth century the problems of the Middle East were compounded by the import of two Western ideas: socialism and secular Arab nationalism, neither of which delivered on their promises of creating prosperous and just societies. By implication Lewis suggests that feelings of humiliation are the animating force behind Al-Qaeda and its affiliates.

Three weeks after the 9/11 attacks, as the United States began launching air strikes against Taliban positions, a videotape of Bin Laden sitting on a rocky outcrop suddenly appeared on Al-Jazeera television. On the tape Bin Laden said, "What America is tasting now is something insignificant compared to what we have tasted for scores of years. [The Islamic world] has been tasting this humiliation and this degradation for eighty years." Bin Laden continued, "Neither America nor the people who live in it will dream of security before we live in it in Palestine, and not before the infidel armies leave the land of Muhammad [the Arabian Peninsula]." So in his first statement following 9/11, Bin Laden emphasized the "humiliation" of the Muslim world and the deleterious effects of American foreign policies in the Middle East.

In this sense, Bin Laden seems to agree with Bernard Lewis' analysis of the problems facing the Islamic world. Indeed, Bin Laden often talks about the "humiliation" suffered by Muslims at the hands of the West. For Bin Laden, the 1916 Sykes-Picot agreement that secretly carved up the Ottoman Empire between the French and British has the same resonance that the 1919 Treaty of Versailles had for Hitler. It must be avenged and reversed. The presence of a large American army in the heart of the Middle East in Iraq over the past three years is seen as ample confirmation among Bin Laden's followers that the West continues to humiliate Muslims.

U.S. foreign policies in the Middle East. By Bin Laden's own account, that's why Al-Qaeda is attacking the United States. His animosity towards the United States has never been driven by a cultural critique. He is silent on the matters of Madonna, Hollywood, homosexuality or drugs in his diatribes against America. By his own account, U.S. support for Israel, in particular the support it gave to Israel's invasion of southern Lebanon in 1982, first triggered Bin Laden's anti-Americanism, which during the 1980s took the form of urging a boycott of American goods. Now, of course, it is the U.S. presence in Iraq that has emerged as the principal rallying cry of the militant jihadists. As there is a bipartisan consensus among American elites on the need for a robust U.S. military presence in the Middle East and for the continued strong support for Israel, American foreign policies will likely continue to provoke resentment in the Muslim world for the foreseeable future.

The alienation of Muslim immigrants in the West. Three out of four of the 9/11 pilots and two key 9/11 planners, KSM and Ramzi bin al-Shibh, became more militant while they were living in the West. It seems that some combination of discrimination, alienation and homesickness turned them all in a more radical direction. And this is true for other anti-Western terrorists. Los Angeles Times researcher Swati Pandey and myself examined the biographies of 79 terrorists responsible for five of the worst anti-Western terrorist attacks in recent memory: the World Trade Center bombing in 1993, the Africa Embassies bombings in 1998, the September 11 attacks, the Bali nightclub bombings in 2002 and last year's London bombings. We found that one in four of the terrorists involved had attended colleges in the West. Similarly, researchers such as Dr. Marc Sageman argue that many terrorists affiliated with Al-Qaeda are either immigrants to the West or second generation Muslims who have not been integrated into their European host countries. For demographic reasons (the native populations of most Western countries are in decline) and for economic reasons (the economies of many Muslim countries are in freefall), there will be an exponentially growing number of Muslim immigrants to the West in the coming years, some of whom will become alienated and adopt Bin Laden's world view.

9/11 does have something to do with a particular reading of Islamic texts. In all the many discussions of the "root causes" of Islamist terrorism, Islam is rarely, if ever, mentioned. This is surprising because if you asked Bin Laden what his war was about, he would answer that it's all about the defense of Islam. This is not to say that Islam is in any way a "bad" or "evil" religion, but on the principle that we should listen to what our enemies are saying, Bin Laden justifies his war based on a corpus of Muslim beliefs and can find enough ammunition in the Quran to give his war a patina of religious legitimacy. For instance, Bin Laden often invokes the "Sword" verses of the Quran, which urge unprovoked (pre-emptive!) attacks on infidels. Of course, that is a selective reading of the Quran as there are other verses that justify only "defensive" jihads, but the point is the Sword verses are in the Quran and therefore are the Word of God. This is not something that apologists can simply wish away. This conviction that they are doing God's will frees Islamist terrorists to conduct mass-casualty attacks of the kind that secular terrorist groups historically never undertook.

9/11 was the collateral damage of a clash within Islam. The view that 9/11 was the result of a conflict within the Muslim world was first brilliantly articulated in November 2001 in Foreign Affairs by Michael Scott Doran. Doran explained that Bin Laden's followers "consider themselves an island of true believers surrounded by a sea of iniquity and think that the future of religion itself, and therefore the world depends on them and their battle." The Egyptians in Al-Qaeda in particular, such as Ayman al-Zawahiri, hold this view, having imbibed it from the works of the jihadist theoretician Sayyid Qutb, who explained that most of the modern Middle East is living in a state of pagan ignorance. This insight animated the Egyptian jihadist belief that they should overthrow the "near enemy" regimes of the Middle East run by "apostate" rulers. It was then the next intellectual step for Bin Laden to urge Zawahiri that the root of the problem was not the "near enemy" but the "far enemy", the United States, which propped up the status quo in the Middle East and was therefore ultimately responsible for the regimes in the region. This analysis is now widely held among militants across the Muslim world.

Authoritarian regimes in the Middle East helped to incubate the militants. It was their experience in the hellish jails of Cairo, for instance, that radicalized both Qutb and Zawahiri. And it is also not an accident that so many key members of Al-Qaeda have been Egyptians and Saudis. However, with the present price of oil it is very unlikely that the Saudi regime will do much to reform politically. Meanwhile, Mubarak's government in Egypt--having made some pretense of reform with the presidential elections in September 2005 and parliamentary elections two months later--is now even more repressive than before 9/11, cracking down on judges, for instance, and imprisoning its political opponents on trumped up charges. The "Arab spring" that was touted by some commentators in 2005 now seems a distant mirage.

There can be no unified-field theory that explains what happened on 9/11; rather it was the confluence of the factors outlined above that help us understand the underlying causes of the attacks on New York and Washington. On 9/11, we were collateral damage in a civil war within the world of political Islam. On the one side there are those like Bin Laden who want to install Taliban-style theocracies from Indonesia to Morocco. On the other side, there is a silent majority of Muslims who are prepared to deal with the West, who do not see the Taliban as a workable model for modern Islamic states, and who reject violence. Bin Laden adopted a war against "the far enemy" in order to hasten the demise of the "near enemy" regimes in the Middle East, so his vision of political Islam could be installed around the Muslim world. And he used 9/11 to advance that cause. That effort has largely failed, but the underlying problems in the Muslim world remain virtually unchanged five years later and will likely provide the fuel for future attacks against us.