Why did the wartime alliance between the us and soviet union unravel?

The wartime alliance between the United States and the Soviet Union began to unravel because the United States felt threatened when Joseph Stalin signed a nonaggression pact with Germany. The wartime alliance between the United States and the Soviet Union began to unravel even before the end of World War II. When the war ended, the Red Army occupied much of Eastern Europe, thus it is believed that origins also play a role in the wartime alliance.