After the Soviet Union collapsed in 1991, the U.S. emerged as the world's sole superpower and, with this, continued interventions in Africa, Eastern Europe, and the Middle East. Following the September 11 attacks in 2001, the U.S. and its NATO allies launched the Global War on Terror in which the U.S. waged international counterterrorism campaigns against various extremist groups such as al-Qaeda and the Islamic State in various countries. The Bush Doctrine of preemptive war saw the U.S. invade Iraq in 2003 and saw the military expand its presence in Africa and Asia via a revamped policy of foreign internal defense.