al-Qaeda, Arabic al-Qāʿidah (“the Base”)Osama bin Laden.AFP/Getty Imagesbroad-based militant Islamist organization founded by Osama bin Laden in the late 1980s.

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. Based in Sudan for a period in the early 1990s, 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 militant Islamist organizations, including Egypt’s Islamic Jihad and the Islamic Group, and on several occasions its leaders declared 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; see USS Cole attack). In 2001, 19 militants associated with al-Qaeda staged the September 11 attacks against the United States. Within weeks the U.S. government responded by attacking Taliban and al-Qaeda forces in Afghanistan. Thousands of militants were killed or captured, among them several key members (including the militant who allegedly planned and organized the September 11 attacks), and the remainder and their leaders were driven into hiding.

The invasion of Afghanistan in 2001 challenged that country’s viability as an al-Qaeda sanctuary and training ground and compromised communication, operational, and financial linkages between al-Qaeda leadership and its militants. Rather than significantly weakening al-Qaeda, however, these realities prompted a structural evolution and the growth of “franchising.” Increasingly, attacks were orchestrated not only from above by the centralized leadership (after the U.S. invasion of Afghanistan, based in the Afghan-Pakistani border regions) but also by the localized, relatively autonomous cells it encouraged. Such grassroots independent groups—coalesced locally around a common agenda but subscribing to the al-Qaeda name and its broader ideology—thus meant a diffuse form of militancy, and one far more difficult to confront.

With this organizational shift, al-Qaeda was linked—whether directly or indirectly—to more attacks in the six years following September 11 than it had been in the six years prior, including attacks in Jordan, Kenya, Saudi Arabia, Indonesia, Turkey, the United Kingdom, Israel, Algeria, and elsewhere. At the same time, al-Qaeda increasingly utilized the Internet as an expansive venue for communication and recruitment and as a mouthpiece for video messages, broadcasts, and propaganda. Meanwhile, some observers expressed concern that U.S. strategy—centred primarily on attempts to overwhelm al-Qaeda militarily—was ineffectual, and at the end of the first decade of the 21st century, al-Qaeda was thought to have reached its greatest strength since the attacks of September 2001.

On May 2, 2011, bin Laden was killed by U.S. military forces after U.S. intelligence located him residing in a secure compound in Abbottabad, Pakistan, 31 miles (50 km) from Islamabad. The operation was carried out by a small team that reached the compound in Abbottabad by helicopter. After bin Laden’s death was confirmed, it was announced by U.S. Pres. Barack Obama, who hailed the operation as a major success in the fight against al-Qaeda. On June 16, 2011, al-Qaeda released a statement announcing that Ayman al-Zawahiri, bin Laden’s long-serving deputy, had been appointed to replace bin Laden as the organization’s leader.