Final answer:
The US-Soviet Union wartime alliance ended due to the breakdown of friendly relations after World War II. This led to the start of the Cold War, a period of rivalry and tension that lasted until the dissolution of the Soviet Union in 1991.
Step-by-step explanation:
The US-Soviet Union wartime alliance ended due to the breakdown of friendly relations between the two countries after World War II. Both nations had different visions for the future of Europe and the world, with the Soviet Union seeking to establish Communist governments in Eastern Europe and spread revolution to other parts of the world, while the United States wanted to expand its influence by protecting or installing democratic governments and promoting free-market economies. This led to the start of the Cold War, a period of rivalry and tension that lasted until the dissolution of the Soviet Union in 1991.