The Soviet Union, officially the Union of Soviet Socialist Republics (USSR), was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union comprising fifteen top-level republics; in practice, both its government and its economy were built on a highly centralized model until its final years. It was a one-party state governed by the Communist Party of the Soviet Union, with Moscow as the capital. Other major cities included Leningrad, Kiev, Minsk, Tashkent, Alma-Ata and Novosibirsk. The Soviet Union was the largest country in the world by land area, covering over 22,402,200 square kilometres (8,649,500 square miles) and spanning eleven time zones.

The Soviet Union traces its origin to the 1917 October Revolution which saw the Bolsheviks led by Vladimir Lenin topple the Provisional Government and establish the RSFSR, the world's first constitutionally socialist state. The October Revolution followed the earlier February Revolution which saw the abdication of Tsar Nicholas II. Bolshevik seizure of power lead to the Russian Civil War, which pitted the Bolsheviks against the White Army. In 1922, the White Army was defeated which spearheaded the creation of the Soviet Union and its Communist Party.

Following Lenin's death and state funeral in 1924, Joseph Stalin assumed leadership over the party and country. Beginning a period of Soviet governance guided by Stalinism he inaugurated rapid industrialization and forced collectivization, which led to significant economic growth but also contributed to the Soviet famine of 19301933. His rule also saw the expansion of the labour camp system under the Gulag. Between 1936 and 1938, Stalin carried out the Great Purge, a campaign of political repression through which he solidified his power. In 1941, Nazi Germany invaded the Soviet Union, opening up the Eastern Front of World War II. The combined Soviet civilian and military casualty figuresestimated to be around 27 million peopleaccounted for the majority of losses on the side of the Allies. The total defeat of the Axis in 1945 marked a formal cessation of hostilities, and the territories taken by Soviet forces subsequently formed various Soviet satellite states.

By 1947, newfound tensions between the Soviet Union and the United States had escalated into the Cold War. During this period, the Soviet-aligned Eastern Bloc confronted the American-aligned Western Bloc. The two sides consolidated their opposition to each other through ideology-based military alliances: the Warsaw Pact, which formed in 1955 to serve Soviet interests, and the North Atlantic Treaty Organization, which formed earlier in 1949 to serve American interests. Following Stalin's death and state funeral in 1953, a process of de-Stalinization was initiated by Nikita Khrushchev. The Soviet Union took an early lead in the Space Race with the first artificial satellite (Sputnik 1), the first human spaceflight (Vostok 1), and the first probe to land on another planet (Venera 7). Throughout the 1970s, there was a dtente in Soviet UnionUnited States relations, but bilateral tensions later worsened due to the Soviet invasion of Afghanistan.

In the mid-1980s, the last Soviet leader, Mikhail Gorbachev, sought to reform the country through his policies of glasnost and perestroika. At the end of the Cold War, various socialist states were overthrown by the Revolutions of 1989, jeopardizing the Warsaw Pact. Unrest across the Eastern Bloc was also accompanied by the outbreak of strong nationalist and separatist movements within the Soviet Union itself. To address the question of the country's future, Gorbachev initiated the 1991 Soviet Union referendumboycotted by the Soviet republics of Lithuania, Latvia, Estonia, Armenia, Georgia, and Moldovathat resulted in the majority of participating citizens voting in favour of the New Union Treaty, which aimed to preserve the Soviet Union as a completely reformed country. Later that year, hardline members of the Communist Party staged the August Coup, which was unsuccessful in overthrowing Gorbachev's government; Boris Yeltsin played a high-profile role in facing down the unrest and the Communist Party was subsequently banned, accelerating the dissolution of the Soviet Union. By December 1991, all of the fifteen Soviet republics had emerged as fully independent post-Soviet states.

The Soviet Union made many social and technological achievements and innovations. It was a founding member of the United Nations and one of the permanent members of the United Nations Security Council. It had the world's second-largest economy while the Soviet Armed Forces comprised the world's largest standing military at their peak, also possessing the world's largest nuclear weapons arsenal. Alongside the United States, the Soviet Union was one of the two superpowers from the end of World War II until its dissolution; it exercised global influence through the Eastern Bloc and various forms of aid to the Third World, and scientific research.

The Winter War, also known as the First Soviet-Finnish War, was a war between the Soviet Union and Finland. The war began with a Soviet invasion of Finland on 30 November 1939, three months after the outbreak of World War II, and ended three-and-a-half months later with the Moscow Peace Treaty on 13 March 1940. Despite superior military strength, especially in tanks and aircraft, the Soviet Union suffered severe losses and initially made little headway. The League of Nations deemed the attack illegal and expelled the Soviet Union from the organisation.

The Soviets made several demands, including that Finland cede substantial border territories in exchange for land elsewhere, claiming security reasons—primarily the protection of Leningrad, 32 km (20 mi) from the Finnish border. When Finland refused, the Soviets invaded. Most sources conclude that the Soviet Union had intended to conquer all of Finland, and use the establishment of the puppet Finnish Communist government and the Molotov–Ribbentrop Pact's secret protocols as evidence of this, while other sources argue against the idea of a full Soviet conquest. Finland repelled Soviet attacks for more than two months and inflicted substantial losses on the invaders while temperatures ranged as low as −43 °C (−45 °F). The battles focused mainly on Taipale in Karelian Isthmus, on Kollaa in Ladoga Karelia and on the Raate Road, in Kainuu, but there were also battles in Salla and Petsamo in Lapland. After the Soviet military reorganized and adopted different tactics, they renewed their offensive in February and overcame Finnish defences.

Hostilities ceased in March 1940 with the signing of the Moscow Peace Treaty in which Finland ceded 9% of its territory to the Soviet Union. Soviet losses were heavy, and the country's international reputation suffered. Their gains exceeded their pre-war demands, and the Soviets received substantial territories along Lake Ladoga and further north. Finland retained its sovereignty and enhanced its international reputation. The poor performance of the Red Army encouraged German Chancellor Adolf Hitler to believe that an attack on the Soviet Union would be successful and confirmed negative Western opinions of the Soviet military. After 15 months of Interim Peace, in June 1941, Germany commenced Operation Barbarossa, and the Continuation War between Finland and the Soviets began.