The collapse of the Soviet Union is inextricably tied to the reformist policies of the then-General Secretary of the Communist Party, Mikhail Gorbachev. Gorbachev came to power in a single-party, multi-ethnic state that ruled a network of satellite countries by force and coercion. By weakening Soviet mechanisms for maintaining power, Gorbachev's reforms undermined the ability of the state to hold onto its possessions and stave off internal challenges.
The proximate cause of the Soviet collapse was the attempt, on Aug. 19, 1991, to overthrow Gorbachev and install a new regime of Communist hardliners. The coup was likely timed to prevent the ratification of a treaty that would have converted the Soviet Union into a European Union-style confederation with less centralized authority. The coup attempt was thwarted when loyal units of the Soviet army, responding to the orders of Russian President Boris Yeltsin, refused to fire on protesters at the Kremlin.
In the months following the abortive coup, one republic after another declared independence from the central government in Moscow, where Gorbachev's authority was weaker than ever. The end came on Dec. 21, when 11 of the 12 republics, excluding Georgia, signed the Alma-Alta Protocol, which recognized the dissolution of the Soviet Union. The delegates at the meeting also accepted Gorbachev's resignation, which had not been offered yet. On Dec. 26, the Soviet Council of Republics voted itself out of existence, and the Soviet Union ceased to exist.