<P> Shortly before World War II, Stalin signed the Molotov--Ribbentrop Pact agreeing to non-aggression with Nazi Germany, after which the two countries invaded Poland in September 1939 . In June 1941, the pact collapsed as Germany turned to attack the Soviet Union, opening the largest and bloodiest theatre of war in history . Soviet war casualties accounted for the highest proportion of the conflict in the effort of acquiring the upper hand over Axis forces at intense battles such as Stalingrad and Kursk . The territories overtaken by the Red Army became satellite states of the Soviet Union; the postwar division of Europe into capitalist and communist halves would lead to increased tensions with the West, led by the United States . </P> <P> The Cold War emerged by 1947, as the Eastern Bloc, united under the Warsaw Pact in 1955, confronted the Western Bloc, united under NATO in 1949 . On 5 March 1953, Stalin died and was quickly succeeded by Nikita Khrushchev, who in 1956 denounced Stalin and began the De-Stalinization of Soviet society through the Khrushchev Thaw . The Soviet Union took an early lead in the Space Race, with the first artificial satellite and the first human spaceflight . Khrushchev was removed from power by his colleagues in 1964 and was succeeded as head of state by Leonid Brezhnev . In the 1970s, there was a brief détente of relations with the United States, but tensions resumed with the Soviet--Afghan War in 1979 . In the mid-1980s, the last Soviet leader, Mikhail Gorbachev, sought to reform and liberalize the economy through his policies of glasnost (government transparency) and perestroika (openness, restructuring). Under Gorbachev, the role of the Communist Party in governing the state was removed from the constitution, causing a surge of severe political instability to set in . The Cold War ended during his tenure, and in 1989, Soviet satellite states in Eastern Europe overthrew their respective communist governments . </P> <P> With the rise of strong nationalist and separatist movements inside the union republics, Gorbachev tried to avert a dissolution of the Soviet Union in the post-Cold War era . A March 1991 referendum, boycotted by some republics, resulted in a majority of participating citizens voting in favor of preserving the union as a renewed federation . Gorbachev's power was greatly diminished after Russian President Boris Yeltsin played a high - profile role in facing down an abortive August 1991 coup d'état attempted by Communist Party hardliners . On 25 December 1991, Gorbachev resigned and the remaining twelve constituent republics emerged as independent post-Soviet states . The Russian Federation--formerly the Russian SFSR--assumed the Soviet Union's rights and obligations and is recognized as the successor state of the Soviet Union . In summing up the international ramifications of these events, Vladislav Zubok stated: "The collapse of the Soviet empire was an event of epochal geopolitical, military, ideological and economic significance ." </P> <Table> <Tr> <Th> Part of a series on the </Th> </Tr> <Tr> <Th> History of Russia </Th> </Tr> <Tr> <Td> </Td> </Tr> <Tr> <Td> <Table> <Tr> <Td> Scythians </Td> <Td> 8th--4th century BCE </Td> </Tr> <Tr> <Td> Sarmatians </Td> <Td> 5th century BCE--4th century CE </Td> </Tr> <Tr> <Td> Early Slavs / Rus' </Td> <Td> pre-9th century </Td> </Tr> <Tr> <Td> Khazar Khaganate </Td> <Td> 7th--10th century </Td> </Tr> <Tr> <Td> Rus' Khaganate </Td> <Td> 9th century </Td> </Tr> <Tr> <Td> Volga Bulgaria </Td> <Td> 9th--13th century </Td> </Tr> <Tr> <Td> Kievan Rus' </Td> <Td> 882--1240 </Td> </Tr> <Tr> <Td> Vladimir - Suzdal </Td> <Td> 1157--1331 </Td> </Tr> <Tr> <Td> Novgorod Republic </Td> <Td> 1136--1478 </Td> </Tr> <Tr> <Td> Mongol Yoke </Td> <Td> 1240s--1480 </Td> </Tr> <Tr> <Td> Grand Duchy of Moscow </Td> <Td> 1283--1547 </Td> </Tr> <Tr> <Td> Tsardom of Russia </Td> <Td> 1547--1721 </Td> </Tr> <Tr> <Td> Russian Empire </Td> <Td> 1721--1917 </Td> </Tr> <Tr> <Td> Russian Republic </Td> <Td> 1917 </Td> </Tr> <Tr> <Td> Russian SFSR </Td> <Td> 1917--1922 </Td> </Tr> <Tr> <Td> Soviet Union </Td> <Td> 1922--1991 </Td> </Tr> <Tr> <Td> Russian Federation </Td> <Td> 1991--present </Td> </Tr> </Table> </Td> </Tr> <Tr> <Td> Timeline </Td> </Tr> <Tr> <Td> Russia portal </Td> </Tr> <Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> </Table>

When did they stop calling russia the soviet union