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 Cold War was a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc, which began following World War II. Historians do not fully agree on its starting and ending points, but the period is generally considered to span the 1947 Truman Doctrine (12 March 1947) to the 1991 dissolution of the Soviet Union (26 December 1991). The term cold war is used because there was no large-scale fighting directly between the two superpowers, but they each supported major regional conflicts known as proxy wars. The conflict was based around the ideological and geopolitical struggle for global influence by these two superpowers, following their temporary alliance and victory against Nazi Germany in 1945. Aside from the nuclear arsenal development and conventional military deployment, the struggle for dominance was expressed via indirect means such as psychological warfare, propaganda campaigns, espionage, far-reaching embargoes, rivalry at sports events and technological competitions such as the Space Race.

The Western Bloc was led by the United States as well as the other First World nations of the Western Bloc that were generally liberal democratic but tied to a network of the authoritarian states, most of which were their former colonies. The Eastern Bloc was led by the Soviet Union and its Communist Party, which had an influence across the Second World and was also tied to a network of authoritarian states. The US government supported anti-communist governments and uprisings across the world, while the Soviet government funded left-wing parties and revolutions around the world. As nearly all the colonial states achieved independence in the period 1945–1960, they became Third World battlefields in the Cold War.

The first phase of the Cold War began shortly after the end of the Second World War in 1945. The United States and its allies created the NATO military alliance in 1949 in the apprehension of a Soviet attack and termed their global policy against Soviet influence containment. The Soviet Union formed the Warsaw Pact in 1955 in response to NATO. Major crises of this phase included the 1948–1949 Berlin Blockade, the 1927–1949 Chinese Civil War, the 1950–1953 Korean War, the 1956 Hungarian Revolution, the 1956 Suez Crisis, the Berlin Crisis of 1961 and the 1962 Cuban Missile Crisis. The US and the USSR competed for influence in Latin America, the Middle East, and the decolonizing states of Africa, Asia, and Oceania.

Following the Cuban Missile Crisis, a new phase began that saw the Sino-Soviet split between China and the Soviet Union complicate relations within the Communist sphere, while France, a Western Bloc state, began to demand greater autonomy of action. The USSR invaded Czechoslovakia to suppress the 1968 Prague Spring, while the US experienced internal turmoil from the civil rights movement and opposition to the Vietnam War. In the 1960s–1970s, an international peace movement took root among citizens around the world. Movements against nuclear weapons testing and for nuclear disarmament took place, with large anti-war protests. By the 1970s, both sides had started making allowances for peace and security, ushering in a period of détente that saw the Strategic Arms Limitation Talks and the US opening relations with the People's Republic of China as a strategic counterweight to the USSR. A number of self-proclaimed Marxist regimes were formed in the second half of the 1970s in the Third World, including Angola, Mozambique, Ethiopia, Cambodia, Afghanistan and Nicaragua.

Détente collapsed at the end of the decade with the beginning of the Soviet–Afghan War in 1979. The early 1980s was another period of elevated tension. The United States increased diplomatic, military, and economic pressures on the Soviet Union, at a time when it was already suffering from economic stagnation. In the mid-1980s, the new Soviet leader Mikhail Gorbachev introduced the liberalizing reforms of glasnost ("openness", c. 1985) and perestroika ("reorganization", 1987) and ended Soviet involvement in Afghanistan in 1989. Pressures for national sovereignty grew stronger in Eastern Europe, and Gorbachev refused to militarily support their governments any longer.

In 1989, the fall of the Iron Curtain after the Pan-European Picnic and a peaceful wave of revolutions (with the exception of Romania and Afghanistan) overthrew almost all communist governments of the Eastern Bloc. The Communist Party of the Soviet Union itself lost control in the Soviet Union and was banned following an abortive coup attempt in August 1991. This in turn led to the formal dissolution of the USSR in December 1991, the declaration of independence of its constituent republics and the collapse of communist governments across much of Africa and Asia. The United States was left as the world's sole superpower.

The Cold War and its events have left a significant legacy. It is often referred to in popular culture, especially with themes of espionage and the threat of nuclear warfare. For subsequent history see International relations since 1989.