The Union of Soviet Socialist Republics (Russian: Сою́з Сове́тских Социалисти́ческих Респу́блик, tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik) abbreviated to USSR (Russian: СССР, translit: SSSR) or shortened to the Soviet Union (Russian: Сове́тский Сою́з, tr. Sovetskij Soyuz), was a Marxist-Leninist state on the Eurasian continent that existed between 1922 and 1991. It was governed as a single-party state by the Communist Party with Moscow as its capital. A union of multiple subnational Soviet republics, its government and economy were highly centralised.
The Soviet Union had its roots in the Russian Revolution of 1917, which overthrew the Russian Empire. The Bolsheviks, the majority faction of the Social Democratic Labour Party, led by Vladimir Lenin, then led a second revolution which overthrew the provisional government and established the Russian Socialist Federative Soviet Republic (renamed Russian Soviet Federative Socialist Republic in 1936), beginning a civil war between pro-revolution Reds and counter-revolution Whites. The Red Army entered several territories of the former Russian Empire, and helped local Communists take power through soviets that nominally acted on behalf of workers and peasants. In 1922, the Communists were victorious, forming the Soviet Union with the unification of the Russian, Transcaucasian, Ukrainian, and Byelorussian republics. Following Lenin's death in 1924, a troika collective leadership and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin suppressed political opposition to him, committed the state ideology to Marxism–Leninism (which he created) and initiated a centrally planned economy. As a result, the country underwent a period of rapid industrialisation and collectivisation which laid the basis for its later war effort and dominance after World War II. However, Stalin established political paranoia, and introduced arbitrary arrests on a massive scale after which the authorities transferred many people (military leaders, Communist Party members, ordinary citizens alike) to correctional labour camps or sentenced them to death.
In the beginning of World War II, after the United Kingdom and France rejected an alliance with the Soviet Union against Nazi Germany, the USSR signed a non-aggression pact with Germany; the treaty delayed confrontation between the two countries, but was disregarded in 1941 when the Nazis invaded, opening the largest and bloodiest theatre of war in history. Soviet war casualties accounted for the highest proportion of the conflict in the cost of acquiring the upper hand over Axis forces at intense battles such as Stalingrad. Soviet forces eventually captured Berlin in 1945, inflicting the vast majority of German losses. The territory overtaken by the Soviet Union from Axis forces in Central and Eastern Europe became satellite states of the Eastern Bloc. Ideological and political differences with Western Bloc counterparts directed by the United States led to the formation of Comecon and the Warsaw Pact, culminating in the prolonged Cold War.
Following Stalin's death in 1953, a period of moderate social and economic liberalization, known as "de-Stalinisation" and "Khrushchev Thaw", occurred under the administration of Nikita Khrushchev. The Soviet Union then went on to initiate significant technological achievements of the 20th century, including launching the first ever satellite and world's first human spaceflight, which led it into the Space Race. The 1962 Cuban Missile Crisis marked a period of extreme tension between the two superpowers, considered the closest to a mutual nuclear confrontation. In the 1970s, a relaxation of relations followed, but tensions resumed when the Soviet Union deployed troops in Afghanistan at the request of its new socialist government in 1979. The campaign drained economic resources and dragged on without achieving meaningful political results.
In the late 1980s the last Soviet leader, Mikhail Gorbachev, sought to reform the Union and move it in the direction of Nordic-style social democracy, introducing the policies of glasnost and perestroika in an attempt to end the period of economic stagnation and democratize the government. However, this led to the rise of strong nationalist and separatist movements. Central authorities initiated a referendum, boycotted by the Baltic republics, Armenia, Georgia, and Moldova, which resulted in the majority of participating citizens voting in favour of preserving the Union as a renewed federation. In August 1991, a coup d'état was attempted by hardliners against Gorbachev, with the intention of reversing his policies. The coup failed, with Russian President Boris Yeltsin playing a high-profile role in facing down the coup, resulting in the banning of the Communist Party. On 25 December 1991, Gorbachev resigned and the remaining twelve constituent republics emerged from the dissolution of the Soviet Union as independent post-Soviet states.
The Russian Federation (formerly the Russian SFSR) assumed the Soviet Union's rights and obligations and is recognised as its continued legal personality.