The leader of the Soviet Union during World War II was Joseph Stalin. He rose to power in the 1920s and became the General Secretary of the Communist Party of the Soviet Union. Under his leadership, the USSR initially signed a non-aggression pact with Nazi Germany in 1939. However, after Hitler’s invasion in June 1941, Stalin led the Soviet Union through a brutal and challenging war on the Eastern Front. His policies, including the massive mobilization of industry and the military, were significant in the eventual defeat of the Axis powers and the establishment of the USSR as a superpower.