Answer:
The United States was Allied with the U.S.S.R,, because they had a common enemy, the Axis Power. That being said, they set aside their differences to fight. After all, there is a saying, "The enemy of my enemy, is my friend".
Why did they become enemies and "fought" the Cold War afterwards (more or less proxy wars) against each other? They didn't have a common enemy anymore, and they were trying to expand their spheres of influence as well as creating alliances and buffer zones to become the world power essentially. In a world, there is no such thing as "two superpowers", but only one. As in history, we see that the U.S.S.R fell apart, leading to the U.S. being the only major power in the world.
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