Joseph Stalin was primarily responsible for the expansion of the Soviet Union following World War II. Rising to power after Lenin's death in 1924, Stalin implemented policies that centralized control over the economy and society while promoting aggressive foreign relations. After WWII, he established Soviet influence in Eastern Europe, creating satellite states in countries such as Poland, Hungary, and East Germany. Stalin's pursuit of communist ideology led to a tense Cold War climate with the West, characterized by political, military, and ideological rivalry. This expansion significantly shaped global geopolitics and impacted international relations for decades.