The Soviet Union (Russian: Советский Союз, tr. Sovetsky Soyuz), officially the Union of Soviet Socialist Republics, was a constitutionally socialist state that existed in Eurasia between 1922 and 1991.
The Soviet Union was a single-party state ruled by the Communist Party from its foundation until 1990. Even though the USSR was technically a union of 15 subnational Soviet republics, its government and economy was highly centralized.
The Russian Revolution of 1917 caused the downfall of the Russian Empire. Following the Russian Revolution, there was a struggle for power between the Bolshevik party, led by Vladimir Lenin, and the anti-communist White movement. In December 1922, the Bolsheviks won the civil war, and the Soviet Union was formed with the merger of the Russian Soviet Federative Socialist Republic, the Transcaucasian Socialist Federative Soviet Republic, the Ukrainian Soviet Socialist Republic and the Byelorussian Soviet Socialist Republic.
Following the death of Vladimir Lenin in 1924, Joseph Stalin took power, leading the USSR through a large-scale industrialization program. Stalin established a planned economy and suppressed political opposition to him and the Communist party.
In June 1941, Nazi Germany and its allies invaded the Soviet Union, breaking the non-aggression pact, which the latter had signed in 1939. After four years of brutal warfare, the Soviet Union emerged victorious as one of the world's two superpowers, the other being the United States.
The Soviet Union and its Eastern European satellite states engaged in the Cold War, a prolonged global ideological and political struggle against the United States and its Western Bloc allies, which it ultimately lost in the face of economic troubles and both domestic and foreign political unrest. In the late 1980s, the last Soviet leader Mikhail Gorbachev tried to reform the state with his policies of perestroika and glasnost, but the Soviet Union collapsed and was formally dissolved in December 1991 after the abortive August coup attempt. The Russian Federation assumed its rights and obligations.
With an area of 22,402,200 square kilometres (8,649,500 sq mi), the Soviet Union was the world's largest state. Covering a sixth of the Earth's land surface, its size was comparable to that of North America. The European portion accounted for a quarter of the country's area, and was the cultural and economic center. The eastern part in Asia extended to the Pacific Ocean to the east and Afghanistan to the south, and was much less populous. It spanned over 10,000 kilometres (6,200 mi) east to west across 11 time zones, and almost 7,200 kilometres (4,500 mi) north to south. It had five climate zones: tundra, taiga, steppes, desert, and mountains.
The Soviet Union had the world's longest border, measuring over 60,000 kilometres (37,000 mi), two-thirds of it a coastline of the Arctic Ocean. Across the Bering Strait was the United States. The Soviet Union bordered Afghanistan, China, Czechoslovakia, Finland, Hungary, Iran, Mongolia, North Korea, Norway, Poland, Romania, and Turkey from 1945 to 1991.
The Soviet Union's longest river was the Irtysh. Its highest mountain was Communism Peak (now Ismail Samani Peak) in Tajikistan, at 7,495 metres (24,590 ft). The world's largest lake, the Caspian Sea, lay mainly within the Soviet Union. The world's largest freshwater and deepest lake, Lake Baikal, was in the Soviet Union.