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Officially the Union of Soviet Socialist Republics (USSR), a nation formerly located in eastern Europe and northwestern Asia. Its capital and largest city was Moscow. In 1917 the Bolsheviks, led by Lenin, seized the government of Russia, and in 1922 Russia merged with the Ukrainian, Belorussian, and Transcaucasian republics to form the USSR. Joseph Stalin emerged as the Soviet leader after Lenin's death in 1924. Under Stalin, the 1930s were marked by political repression and terror (see Stalin's Purge Trials). After the Nazi-Soviet Non-Aggression Pact of 1939, the Soviet Union added parts of Finland, Poland, and Romania to its territory and annexed the Baltic republics of Estonia, Latvia, and Lithuania. Invaded by Germany in 1941, the Soviet Union suffered vast losses but emerged from World War II on the winning side and soon became a nuclear superpower. Postwar American-Soviet relations saw the start of the cold war, as the Soviet Union extended its control over the Eastern Bloc. The Cuban missile crisis was provoked by the buildup of Soviet missiles in Cuba. In the 1970s the Soviet Union entered a period of ��é�ٱ�Գٱ� with the United States. The reforms (glasnost and perestroika) introduced by Mikhail Gorbachev weakened the Communist party's control, which suffered a mortal blow when hard-liners tried unsuccessfully in 1991 to overthrow Gorbachev. As Communist dominance faded, nationalism rose within the republics that made up the Soviet Union. The Baltic republics of Latvia, Estonia, and Lithuania, Armenia, Belarus, Georgia, Moldova, and various republics of the Caucasus Mountains — Azerbaijan, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan — declared their independence. The Soviet Union was formally dissolved in 1991. A loose federation, known as the Commonwealth of Independent States and made up of some former Soviet republics, succeeded it, but the Commonwealth is not recognized as a nation. Russia took the former Soviet Union's seat on the Security Council of the United Nations.