The Union of Soviet Socialist Republics (USSR), commonly known as the Soviet Union, was a socialist state in Eurasia that existed from 30 December 1922 to 26 December 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 (Russian SFSR). Other major urban centres were Leningrad, Kiev, Minsk, Alma-Ata, and Novosibirsk.
Extending across the entirety of Northern Asia and much of Eastern Europe, the Soviet Union had spanned eleven time zones and incorporated a wide range of environments and landforms. From northwest to southeast, the Soviet Union shared land borders with Norway, Finland, Poland, Czechoslovakia, Hungary, Romania, Turkey, Iran, Afghanistan, China, Mongolia, and North Korea. It shared its maritime borders with Japan by the Sea of Okhotsk and the US state of Alaska across the Bering Strait. With an area of 22,402,200 square kilometres (8,649,500 sq mi), the Soviet Union was the largest country in the world by area, covering more than one-eighth of the Earth's inhabited land area, and the third most populous, with over 288 million people as of 1989, with 80% of the population living in the western, European part of the country.
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 by the Treaty on the Creation of the USSR which legalized the unification of the Russian, Transcaucasian, Ukrainian and Byelorussian republics that had occurred from 1918. 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 constructed a command economy which led to a period of rapid industrialization and collectivization. During this period of totalitarian rule, political paranoia fermented and 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 over 600,000 deaths. Suppression of political critics and forced labor were carried out by Stalin's government. In 1933, a major famine that became known as the Holodomor in Soviet Ukraine struck multiple Soviet grain-growing regions, causing the deaths of some 3 to 7 million people.
In August 1939, days before the start of World War II, the Soviets signed the Molotov–Ribbentrop Pact agreeing to non-aggression with 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 and the postwar division of Europe into capitalist and communist halves would lead to increased tensions with the West, led by the United States of America.
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 eventually 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, the first human spaceflight, and successfuly launched the first and so far the only probe to planet Venus. Dissatisfied with Khrushchev's policies, the Communist Party's conservative wing led a coup d'état against Khrushchev in 1964, quietly ousting him without any bloodshed. In the early 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 (openness) and perestroika (restructuring). 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. 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. 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 on 26 December 1991, the Supreme Soviet of the Soviet Union met and formally dissolved the Soviet Union, thereby ending the Cold War, 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".
Throughout its existence, the Soviet Union was a powerhouse of many significant technological achievements and innovations of the 20th century, including the world's first human-made satellite and the launching of the first humans in space. The country had the world's second largest economy and the largest standing military in the world.. The Soviet Union was recognized as one of the five 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), the World Federation of Trade Unions (WFTU) and the leading member of the Council for Mutual Economic Assistance (CMEA) and the Warsaw Pact.