The Soviet Union collapsed and dissolved in 1991. Its formal end came on December 26, 1991, when the Soviet of the Republics declared that the Union no longer existed as a sovereign state.
The breakup followed years of economic stagnation, costly military commitments, rising nationalist movements, and political reforms introduced by leader Mikhail Gorbachev. His policies of glasnost, or openness, and perestroika, or restructuring, loosened central control but also exposed deep problems within the Soviet system.
A decisive step came on December 8, when the leaders of Russia, Ukraine, and Belarus signed the Belavezha Accords, declaring that the Soviet Union had ceased to exist and creating the Commonwealth of Independent States. Gorbachev resigned as president on December 25, and the Soviet flag was lowered from the Kremlin that evening.
The date is sometimes confused with earlier milestones, including the failed August 1991 coup and the independence declarations of individual republics. Those events accelerated the collapse, but 1991 is the accepted year of the Soviet Union's dissolution.