Joseph Stalin's authoritarian rule of the Soviet Union and commitment to spreading communism caused significant tensions with the United States during World War II and the onset of the Cold War. Key sources that provide insight into Stalin's actions and the perspectives of Russians living under his rule, as well as the ideological differences between the Soviet Union and the US that led to the emergence of the Cold War in the postwar period. Stalin's oppressive policies at home and distrust of the West abroad were major factors that increased tensions between the two superpowers.