War in Afghanistan: The 20-Year War between the Taliban and the United States Explained

The terrorist attacks of September 11, 2001, fundamentally transformed American foreign policy and set in motion a chain of events that would lead to the longest war in United States history, as the shocking destruction of the World Trade Center and the attack on the Pentagon created an overwhelming demand for retribution while revealing the vulnerability of even the world's most powerful nation to asymmetric threats from non-state actors operating from failed states. The immediate focus on Afghanistan as the sanctuary for Al-Qaeda and the staging ground for the attacks reflected both accurate intelligence about the terrorist organization's presence there and the broader American need to respond decisively to an unprecedented attack on the homeland that had shattered assumptions about geographic security and military invulnerability that had persisted since the end of the Cold War.

The relationship between Al-Qaeda and the Taliban government that controlled most of Afghanistan at the time of the September 11 attacks represented a complex alliance of convenience that would prove fateful for both organizations and for the Afghan people who would bear the costs of the international intervention that followed. Osama bin Laden had established his presence in Afghanistan during the 1990s following his expulsion from Sudan, finding in the Taliban's strict Islamic emirate a sanctuary where he could train operatives, plan attacks, and build the infrastructure necessary for global terrorist operations. The Taliban, led by Mullah Mohammed Omar, had emerged from the chaos of Afghanistan's civil war during the 1990s as a Pashtun-dominated movement that promised to restore order and implement pure Islamic governance, but their harsh interpretation of Islamic law and systematic oppression of women and minorities had left them internationally isolated and dependent on Pakistani support.