The Soviet Union (Russian: Сове́тский Сою́з, Sovetsky Soyuz), officially the Union of Soviet Socialist Republics (Russian: Сою́з Сове́тских Социалисти́ческих Респу́блик, Soyuz Sovetskikh Sotsialisticheskikh Respublik), abbreviated as USSR (Russian: СССР, SSSR), was a socialist state in Eurasia that existed from 1922 to 1991. Nominally a union of multiple national Soviet republics, its government and economy were highly centralized. The country was a one-party state, governed by the Communist Party with Moscow as its capital in its largest republic, the Russian Soviet Federative Socialist Republic. The Russian nation had constitutionally equal status among the many nations of the union but exerted de facto dominance in various respects. Other major urban centers were Leningrad, Kiev, Minsk, Alma-Ata and Novosibirsk. The Soviet Union was one of the five recognized nuclear weapons states and possessed the largest stockpile of weapons of mass destruction. It was a founding permanent member of the United Nations Security Council, as well as a member of the Organization for Security and Co-operation in Europe (OSCE) and the leading member of the Council for Mutual Economic Assistance (CMEA) and the Warsaw Pact.
The Soviet Union had its roots in the October Revolution of 1917, when the Bolsheviks, led by Vladimir Lenin, overthrew the Russian Provisional Government which had replaced Tsar Nicholas II during World War I. In 1922, the Soviet Union was formed with the unification of the Russian, Transcaucasian, Ukrainian and Byelorussian republics. Following Lenin's death in 1924 and a brief power struggle, Joseph Stalin came to power in the mid-1920s. Stalin committed the state's ideology to Marxism–Leninism (which he created) and initiated a centrally planned economy which led to a period of rapid industrialization and collectivization. During this period of totalitarian rule, political paranoia fermented; the late-1930s Great Purge removed Stalin's opponents within and outside of the party via arbitrary arrests and persecutions of many people, resulting in an estimated 600,000 deaths. Suppression of political critics, forced labor and famines were carried out by Stalin's government; in 1933, a major famine struck Soviet Ukraine, causing the deaths of some 3 to 7 million people.
Shortly before World War II, Stalin signed the Molotov–Ribbentrop Pact agreeing to non-aggression with Nazi Germany, after which the two countries invaded Poland in September 1939. In June 1941, the pact collapsed as Germany turned to attack the Soviet Union, opening the largest and bloodiest theatre of war in history. Soviet war casualties accounted for the highest proportion of the conflict in the effort of acquiring the upper hand over Axis forces at intense battles such as Stalingrad and Kursk. The territories overtaken by the Red Army became satellite states of the Soviet Union; the postwar division of Europe into capitalist and communist halves would lead to increased tensions with the West, led by the United States.
The Cold War emerged by 1947, as the Eastern Bloc, united under the Warsaw Pact in 1955, confronted the Western Bloc, united under NATO in 1949. On March 5th 1953, Stalin died and was quickly succeeded by Nikita Khrushchev, who in 1956 denounced Stalin and began the De-Stalinization of Soviet society through the Khrushchev Thaw. The Soviet Union took an early lead in the Space Race, with the first artificial satellite and the first human spaceflight. Khrushchev was removed from power by his colleagues in 1964 and was succeeded as head of state by Leonid Brezhnev. In the 1970s, there was a brief détente of relations with the United States, but tensions resumed with the Soviet–Afghan War in 1979. In the mid-1980s, the last Soviet leader, Mikhail Gorbachev, sought to reform and liberalize the economy through his policies of glasnost and perestroika. Under Gorbachev, the role of the Communist Party in governing the state was removed from the constitution, causing a surge of severe political instability to set in. The Cold War ended during his tenure, and in 1989, Soviet satellite states in Eastern Europe overthrew their respective communist governments.
With the rise of strong nationalist and separatist movements inside the union republics, Gorbachev tried to avert a dissolution of the Soviet Union in the post-Cold War era. A March 1991 referendum, boycotted by some republics, resulted in a majority of participating citizens voting in favor of preserving the union as a renewed federation. Gorbachev's power was greatly diminished after Russian President Boris Yeltsin played a high-profile role in facing down an abortive August 1991 coup d'état attempted by Communist Party hardliners. On December 25th 1991, Gorbachev resigned and the remaining twelve constituent republics emerged as independent post-Soviet states. The Russian Federation—formerly the Russian SFSR—assumed the Soviet Union's rights and obligations and is recognized as the successor state of the Soviet Union. In summing up the international ramifications of these events, Vladislav Zubok stated: "The collapse of the Soviet empire was an event of epochal geopolitical, military, ideological and economic significance."