The military history of the Soviet Union began in the days following the 1917 October Revolution that brought the Bolsheviks to power. In 1918 the new government formed the Red Army, which then defeated its various internal enemies in the Russian Civil War of 1917–22. The years 1918–21 saw defeats for the Red Army in the Polish–Soviet War (1919–21) and in independence wars for Estonia (1918–20), Latvia (1918–20) and Lithuania (1918–19). The Red Army invaded Finland (November 1939); fought the Battles of Khalkhin Gol of May–September 1939 (together with its ally Mongolia) against Japan and its client state Manchukuo; it was deployed when the Soviet Union, in agreement with Nazi Germany, took part in the invasion of Poland in September 1939, and occupied the Baltic States (June 1940), Bessarabia (June–July 1940) and Northern Bukovina (June–July 1940) (from Romania). In World War II the Red Army became a major military force in the defeat of Nazi Germany and conquered Manchuria. After the war, it occupied East Germany and many nations in central and eastern Europe, which became satellite states in the Soviet bloc. Following the Allies' victory over Germany and Japan in 1945, the Soviet Union became the sole superpower rival to the United States. The Cold War between the two nations led to military buildups, the nuclear arms race, and the Space Race. By the early 1980s the Soviet armed forces had more troops, tanks, artillery guns and nuclear weapons than any other nation on earth. The Soviet Union fell in 1991, not because of military defeat but because of economic and political factors - see History of the Soviet Union (1982–91). The Soviet military consisted of five armed services - in their official order of importance: the Strategic Rocket Forces (service established in 1959) Ground Forces Air Forces Air Defense Forces (separate service from 1949) Naval Forces Two other Soviet militarized forces existed: the Internal Troops (MVD Troops), subordinated to the Ministry of the Interior, and the Border Troops, subordinated to the KGB. Tsarist and revolutionary background The February Revolution replaced the Tsar with the Russian Provisional Government, 1917 which was itself overthrown by the Bolshevik Revolution of 1917. The Russian army, exhausted by its participation in World War I, was in the final stages of disintegration and collapse. Even though Bolshevik influence in the ranks was strong, the officer corps was staffed with many who violently opposed communism.