Al-Qaeda began as a logistical network to support Muslims fighting against the Soviet Union during the Afghan War; members were recruited throughout the Islamic world. When the Soviets withdrew from Afghanistan in 1989, the organization dispersed but continued to oppose what its leaders considered corrupt Islamic regimes and foreign (i.e., U.S.) presence in Islamic lands. With active members and sympathizers in dozens of countries (many veterans of the Afghan conflict), the group eventually reestablished its headquarters in Afghanistan (c. 1996) under the patronage of the Taliban militia.

Al-Qaeda merged with a number of other Islamic extremist organizations, including Egypt's Islamic Jihad and the Islamic Group, and on several occasions its leaders declared jihad (holy war) against the United States. The organization established camps for Muslim militants from throughout the world, training tens of thousands in paramilitary skills, and its agents engaged in numerous terrorist attacks, including the destruction of the U.S. embassies in Nairobi, Kenya, and Dar es Salaam, Tanzania (1998), and a suicide bomb attack against the U.S. warship Cole in Aden, Yemen (2000). In 2001, 19 militants associated with al-Qaeda staged the September 11 attacks against the United States. The American government responded by attacking Taliban and al-Qaeda forces in Afghanistan, killing and capturing thousands of militants and driving the remainder and their leaders into hiding. Despite the subsequent capture of several of its key members (including the militant who allegedly planned and organized the September 11 attacks), al-Qaeda and its sympathizers purportedly continued to stage acts of terrorism throughout the world.