Afghanistan War, international conflict in Afghanistan beginning in 2001 that was triggered by the September 11 attacks and consisted of three phases. The first phase—toppling the Taliban (the ultraconservative political and religious faction that ruled Afghanistan and provided sanctuary for al-Qaeda, perpetrators of the September 11 attacks)—was brief, lasting just two months. The second phase, from 2002 until 2008, was marked by a U.S. strategy of defeating the Taliban militarily and rebuilding core institutions of the Afghan state. The third phase, a turn to classic counterinsurgency doctrine, began in 2008 and accelerated with U.S. Pres. Barack Obama’s 2009 decision to temporarily increase the U.S. troop presence in Afghanistan.
The larger force was used to implement a strategy of protecting the population from Taliban attacks and supporting efforts to reintegrate insurgents into Afghan society. The strategy came coupled with a timetable for the withdrawal of the foreign forces from Afghanistan; beginning in 2011, security responsibilities would be gradually handed over to the Afghan military and police. The new approach largely failed to achieve its aims. Insurgent attacks and civilian casualties remained stubbornly high, while many of the Afghan military and police units taking over security duties appeared to be ill-prepared to hold off the Taliban. By the time the U.S. and NATO combat mission formally ended in December 2014, the 13-year Afghanistan War had become the longest war ever fought by the United States.