The Soviet Union, officially the Union of Soviet Socialist Republics (USSR), was a country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national republics; in practice, both its government and its economy were highly centralized until its final years. It was a one-party state governed by the Communist Party of the Soviet Union, with the city of Moscow serving as its capital within its largest and most populous republic: the Russian SFSR. Other major cities included Leningrad (Russian SFSR), Kiev (Ukrainian SSR), Minsk (Byelorussian SSR), Tashkent (Uzbek SSR), Alma-Ata (Kazakh SSR), and Novosibirsk (Russian SFSR). It was the largest country in the world, covering over 22,402,200 square kilometres (8,649,500 sq mi) and spanning eleven time zones.

The country's roots lay in the October Revolution of 1917, when the Bolsheviks, under the leadership of Vladimir Lenin, overthrew the Russian Provisional Government that had earlier replaced the House of Romanov of the Russian Empire. The Bolshevik victory established the Russian Soviet Republic, the world's first constitutionally guaranteed socialist state. Persisting internal tensions escalated into the Russian Civil War, which saw fighting between the Bolshevik Red Army and many anti-Bolshevik forces across the former Russian Empire, among whom the largest faction was the White Guard. The anti-communist White Guard violently repressed the Bolsheviks as well as suspected "worker and peasant" Bolsheviks during the White Terror. However, the Red Army expanded and helped local Bolsheviks take power, establishing soviets and repressing their political opponents as well as rebellious peasants during the Red Terror. By 1922, the balance of power had shifted and the Bolsheviks had emerged victorious, forming the Soviet Union with the unification of the Russian, Transcaucasian, Ukrainian, and Byelorussian republics. Upon the conclusion of the Russian Civil War, Lenin's government introduced the New Economic Policy, which led to the partial return of a free market and private property; this resulted in a period of economic recovery.

Following Lenin's death in 1924, Joseph Stalin came to power. Stalin suppressed all political opposition to his rule inside the Communist Party and inaugurated a command economy. As a result, the country underwent a period of rapid industrialization and forced collectivization that led to significant economic growth, but also contributed to a man-made famine in 19301933. Additionally, the labour camp system of the Gulag was also expanded in this period. Stalin fomented political paranoia and conducted the Great Purge to remove his actual and perceived opponents from the Communist Party through mass arrests of military leaders, party members, and ordinary citizens alike; all of whom were then sent to correctional labour camps or sentenced to death.

On 23 August 1939, the Soviets signed the MolotovRibbentrop Pact with Nazi Germany, which established an understanding of neutrality and non-aggression between the two sides. With the outbreak of World War II following the German invasion of Poland, the formally neutral Soviet Union invaded and annexed the territories of several states in Eastern Europe, including the eastern regions of Poland, and Lithuania, Latvia, and Estonia. In June 1941, Germany broke the bilateral non-aggression pact and launched a large-scale invasion of the Soviet Union, opening the Eastern Front of the global conflict. Despite initial German successes, the Soviets gained the upper hand over Axis forces at the Battle of Stalingrad and eventually captured Berlin, declaring victory over Germany on 9 May 1945. The combined Soviet civilian and military casualty countestimated to be around 27 million peopleaccounted for the majority of losses on the side of the Allied forces. In the aftermath of World War II, the territory taken by the Red Army formed various Soviet satellite states under the Eastern Bloc. The subsequent beginning of the Cold War in 1947 saw the Eastern Bloc of the Soviet Union confront the Western Bloc of the United States, with the latter grouping becoming largely united in 1949 under the North Atlantic Treaty Organization and the former grouping becoming largely united in 1955 under the Warsaw Pact.

Following Stalin's death in 1953, a period known as de-Stalinization and the Khrushchev Thaw occurred under the leadership of Nikita Khrushchev. The Soviet Union developed rapidly, as millions of peasants were moved into industrialized cities. As part of the Cold War, the Soviets took an early lead in the Space Race with the first artificial satellite, the first human spaceflight, and the first probe to land on another planet (Venus). In the 1970s, there was a brief dtente in the Soviet Union's relationship with the United States, but tensions resumed following the Soviet invasion of Afghanistan in 1979. Lasting until 1989, the SovietAfghan War drained Soviet economic resources and was matched by an escalation of American military aid to the Afghan mujahideen.

In the mid-1980s, the last Soviet leader, Mikhail Gorbachev, sought to further reform and liberalize the economy through his policies of glasnost and perestroika. The goal was to preserve the Communist Party while reversing the Era of Stagnation. In 1989, during the closing stages of the Cold War, various countries of the Warsaw Pact overthrew their MarxistLeninist regimes, which was accompanied by the outbreak of strong nationalist and separatist movements across the entire Soviet Union. In 1991, Gorbachev initiated a national referendumboycotted by the Soviet republics of Lithuania, Latvia, Estonia, Armenia, Georgia, and Moldovathat resulted in the majority of participating citizens voting in favour of preserving the country as a renewed federation. In August 1991, hardline members of the Communist Party staged a coup d'tat against Gorbachev; the attempt failed, with Boris Yeltsin playing a high-profile role in facing down the unrest, and the Communist Party was subsequently banned. The Soviet republics, led by Russia and Ukraine, formally declared independence. On 25 December 1991, Gorbachev resigned from his presidency. All of the republics emerged from the dissolution of the Soviet Union as fully independent post-Soviet states. Above the other former republics, the Russian Federation (formerly the Russian SFSR) assumed the Soviet Union's rights and obligations and has since remained recognized as its successor legal personality in international affairs.

The Soviet Union produced many significant social and technological achievements and innovations, particularly with regard to military power. It boasted the world's second-largest economy, and the Soviet Armed Forces comprised the largest standing military in the world. An NPT-designated state, it possessed the largest arsenal of nuclear weapons in the world. It was a founding member of the United Nations as well as one of the five permanent members of the United Nations Security Council; it was also a member of the OSCE and the WFTU, and the leading member of the Council for Mutual Economic Assistance.

Between the end of World War II in 1945 and its dissolution in 1991, the Soviet Union had maintained its status as one of two superpowers vis--vis the United States. It was sometimes referred to informally as the "Soviet Empire" in relation to its exercising of hegemony across Europe as well as worldwide with a combination of military and economic strength; proxy conflicts and influence in the Third World; and funding of scientific research, especially in space technology and weaponry.

Diplomacy refers to spoken or written speech acts by representatives of states (such as leaders and diplomats) intended to influence events in the international system.Diplomacy is the main instrument of foreign policy and global governance which represents the broader goals and strategies that guide a state's interactions with the rest of the world. International treaties, agreements, alliances, and other manifestations of international relations are usually the result of diplomatic negotiations and processes. Diplomats may also help shape a state by advising government officials.

Modern diplomatic methods, practices, and principles originated largely from 17th-century European custom. Beginning in the early 20th century, diplomacy became professionalized; the 1961 Vienna Convention on Diplomatic Relations, ratified by most of the world's sovereign states, provides a framework for diplomatic procedures, methods, and conduct. Most diplomacy is now conducted by accredited officials, such as envoys and ambassadors, through a dedicated foreign affairs office. Diplomats operate through diplomatic missions, most commonly consulates and embassies, and rely on a number of support staff; term diplomat is thus sometimes applied broadly to diplomatic and consular personnel and foreign ministry officials.