KABUL, July 2 (Reuters) - The departure of U.S. troops from Afghanistan"s Bagram air base on Friday marks the end of a 20-year war. Here are some facts about the Soviet-built air strip, which has served as the main base for two super powers that waged campaigns to subdue Islamist insurgents in Afghanistan. The airfield, built in the 1950s against the backdrop of snow-capped mountains and once visited by President Dwight Eisenhower in 1959, became a vital military hub for the Soviet Union after it invaded Afghanistan in 1979. The Soviets withdrew in 1989. By the 1990s, the abandoned air strip and its bombed-out hangars and watchtowers had become a frontline in a war between the Taliban who held Kabul to the south, and the Northern Alliance fighters based in mountain gorges to its north. After the Sept. 11, 2001 attacks, the United States bombed Taliban positions, allowing the Northern Alliance to sweep south into Kabul. U.S. forces quickly occupied the air strip, using it as the Soviets had before them as their main base in the country. In the early years of the war under President George W. Bush, the CIA used Bagram as a "black site" detention centre for terrorism suspects, subjecting them to abuse that President Barack Obama would later acknowledge as torture. Later, as the U.S. and NATO presence in Afghanistan grew, so did the base. A second runway was built, with permanent barracks, gyms and a Pizza Hut. By 2007, Bagram, where temperatures can drop to -29 degrees Celsius (-20 degrees F), had become a huge base, with three rings of security, processing arriving troops before they were flown to frontline positions. U.S. presidents visited frequently to meet the troops, most recently Donald Trump, who dropped in for Thanksgiving in 2019. Robin Williams, Jay Leno and Kid Rock were among the celebrities who visited over the years. In 2007, while then-Vice President Dick Cheney was in the country, a suicide bomber struck Bagram, killing up to 23 people and injuring 20.