The Soviet Union was formed in 1922, five years after the collapse of the Russian Empire, with the former revolutionary Vladimir Lenin as its first leader. It operated under a one-party system in which the Communist Party governed in congruence with the Premier of the Soviet Union. The Soviet Union ultimately comprised 15 republics across eastern Europe and central Asia: Armenia, Azerbaijan, Belorussia, Estonia, Georgia, Kazakhstan, Kirgiziya, Latvia, Lithuania, Moldavia, Russia, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan. At its height there were nearly 300 million people living under Soviet rule. It transformed into a world power under Joseph Stalin, who led the Soviets to victory on the eastern front in World War II. Post-war struggle for geopolitical control led to the Cold War, a nuclear arms race between the United States and the Soviet Union, which began in 1947 and ended in 1991 with the collapse of the Soviet Union.