Why did 9/11 happen?
The September 11 attacks were carried out by al-Qaeda, motivated by Osama bin Laden's ideology opposing American military presence in Saudi Arabia, U.S. support for Israel, and Western influence in Muslim-majority countries. The attacks were enabled by intelligence failures, exploitable airport security gaps, and al-Qaeda's years of planning from its base in Taliban-controlled Afghanistan.
Understanding why 9/11 happened requires examining the motivations of its perpetrators, the geopolitical conditions that produced them, and the specific failures that allowed the attacks to succeed — without conflating explanation with justification.
Al-Qaeda's ideology drew on a radical interpretation of Islam that framed the Muslim world as under siege by Western — particularly American — cultural, military, and economic power. Osama bin Laden articulated specific grievances: the presence of American military bases in Saudi Arabia (home of Islam's holiest sites), U.S. support for Israel against Palestinians, American-backed sanctions against Iraq that caused widespread civilian suffering, and support for authoritarian Arab governments that suppressed Islamist movements. These were political grievances wrapped in religious language.
Bin Laden's radicalism was itself partly a product of the Cold War. During the 1980s, the United States had supported the mujahideen resistance against the Soviet invasion of Afghanistan, channeling weapons and money through Pakistan's intelligence services. Bin Laden, a wealthy Saudi, had gone to Afghanistan to support the jihad. The experience created a network of battle-hardened fighters and the conviction that a superpower could be defeated. When the Soviet Union withdrew in 1989, these fighters — and this conviction — did not disappear. They metastasized into a global jihadist movement.
The specific planning of the 9/11 attacks began in the late 1990s. Khalid Sheikh Mohammed conceived the operation, and bin Laden approved and funded it. Nineteen hijackers, mostly Saudi nationals, entered the United States legally, trained at American flight schools, and exploited security systems designed to prevent hijackings from an earlier era — when hijackers typically demanded ransom or political concessions, not suicide missions.
Intelligence failures were significant. The CIA knew that al-Qaeda members had entered the United States but failed to share this information with the FBI. Warnings from field agents about suspicious flight school students were not escalated. The institutional inability of American intelligence agencies to share information and connect dots — the 'wall' between foreign and domestic intelligence — was a systemic failure that the 9/11 Commission later documented in detail.
The attacks happened because a small group of determined individuals, motivated by a radical ideology forged in specific geopolitical conditions, found vulnerabilities in a system that had not imagined this particular form of catastrophic attack. The response — two decades of war, expanded surveillance, and the reshaping of global security — has been at least as consequential as the attacks themselves.