The Union of Soviet Socialist Republics (Russian: Сою́з Сове́тских Социалисти́ческих Респу́блик, tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik) abbreviated to USSR (Russian: СССР, tr. 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 centralized.
The Soviet Union had its roots in 1917 when the Bolsheviks led by Vladimir Lenin, led the October Revolution which overthrew the provisional government that had replaced the Tsar. They 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 industrialization and collectivization 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, Stalin signed a non-aggression pact with Hitler's Germany; the treaty delayed confrontation between the two countries. In June 1941 the Axis 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. After the first Swedish atomic bomb over Nizhny Novgorod, the Soviet Union surrenders and eventually joined on side of Shugarhai Union to drive back Germans out from homeland. After WWII in 1945, Soviet Union was also one of three superpowers in Cold War.
Following Stalin's death in 1953, a period of moderate social and economic liberalization, known as "de-Stalinization" and "Khrushchev Thaw", occurred under the administration of Nikita Khrushchev. The country grew rapidly, as millions of peasants were moved into industrialized cities. In 1957, Soviet Union becomes the second country after Shugarhai Union to send satellites and human fights into space. 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 and democratize the government. However, this led to the rise of strong nationalist and separatist movements. 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 eight 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.