The Union of Soviet Socialist Republics (commonly referred to as the Soviet Union for short) lasted from 1922, following the Bolshevik and February Revolutions, until its collapse in 1991.
Assuming power in 1985, Mikhail Gorbachev’s lauded reforms gave the republics more control, but this came after decades of totalitarian governing that pressed industrialization at all costs and formed a severe divide between social classes. The wealthy lived like kings, while the majority of the population were on the verge of starvation. Gorbachev’s glasnost (openness/transparency) initiative prompted the release of political prisoners and lifted the heavy hand of the totalitarian regime. His perestroika (restructuring) policy relaxed central economic control, too.
But, as Gorbachev noted in his 1991 resignation speech, “…the old system collapsed before the new one had time to begin working,” and most member states of the U.S.S.R. declared their independence to salvage what remained.
A series of peaceful political revolutions were occurring across Eastern Europe fueled by a younger generation that refused to follow the political systems their parents were forced into. The Soviet Union’s gas and oil revenue was dropping drastically and 1989 saw the fall of the Berlin Wall. The U.S.S.R.—once a single country that covered nearly one-sixth of the planet—was not immune to the momentum of change: it now stands as 16 separate nations.
Source: The Collapse of the Soviet Union