War in Afghanistan began on 27 April 1978, when the People's Democratic Party of Afghanistan (PDPA) took power in a military coup, known as the Saur Revolution. Most of Afghanistan subsequently experienced uprisings against the PDPA government. The Soviet Union invaded Afghanistan in December 1979 to replace the existing communist government. Afghanistan's resistance forces, known as the mujahideen, fought against the Soviet invasion of Afghanistan. Some factions received support by the United States, with the Pakistani ISI serving as the U.S. middleman, and Saudi Arabia. The Soviet Union had to withdraw its troops in February 1989. The Soviet-backed Afghan communist regime survived for three more years until the fall of Kabul in 1992. The Taliban emerged as a new faction threatening Kabul. Following the September 11 attacks inside the United States in 2001, NATO intervened in Afghanistan under Operation Enduring Freedom. The purpose of this was to defeat Al-Qaeda, remove the Taliban from power and create a viable democratic state.