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 5 March 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 25 December 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."
The word “Soviet” is derived from a Russian word meaning council, assembly, advice, harmony, concord, and all ultimately deriving from the Proto-Slavic verbal stem of *vět-iti "to inform", related to Slavic "věst" ("news"), English "wise", the root in "ad-vis-or" (which came to English through French), or the Dutch "weten" (to know; cf. "wetenschap"=science). The word "sovietnik" means councillor.
A number of organizations in Russian history were called "council" (Russian: сове́т). For example, in the Russian Empire, the State Council, which functioned from 1810 to 1917, was referred to as a Council of Ministers after the revolt of 1905.
During the Georgian Affair, Vladimir Lenin envisioned an expression of Great Russian ethnic chauvinism by Joseph Stalin and his supporters, calling for these nation-states to join Russia as semi-independent parts of a greater union, which he initially named as the Union of Soviet Republics of Europe and Asia (Russian: Сою́з Сове́тских Респу́блик Евро́пы и А́зии, tr. Soyúz Sovétskikh Respúblik Evrópy i Ázii). Stalin initially resisted the proposal, but ultimately accepted it, although – with Lenin's agreement – he changed the name of the newly proposed state to the Union of Soviet Socialist Republics, although all the republics began as Socialist Soviet and did not change to the other order until 1936. In addition, in the national languages of several republics the word "Council/Conciliar" in the respective language was only quite late changed to an adaptation of the Russian "Soviet" – and never in others, e.g., Ukraine.