Twenty years ago, on December 25, 1991, Mikhail Gorbachev resigned as president of the Soviet Union, declaring the office extinct and dissolving the Union of Soviet Socialist Republics, a massive communist empire that had existed since 1922. The USSR had been in a long economic stagnation when Gorbachev came to power in 1985. In order to bring about change, he introduced several reforms, including perestroika and glasnost (openness). Glasnost opened the floodgates of protest and many republics made moves toward independence, threatening the continued existence of the USSR. In August of 1991, a group of Communist Party hardliners frustrated by the separatist movement attempted to stage a coup. They quickly failed due to a massive show of civil resistance -- but the already-faltering government was destabilized even further by the attempt. By December of 1991, 16 Soviet republics had declared their independence, and Gorbachev handed over power to Russian president Boris Yeltsin, ending the USSR. Collected here are photos from those tumultuous months 20 years ago.