Why did the United Sates and the Soviets end up on opposite sides of the Cold War

The US and the Soviet Union not only ended the Cold War on opposite sides, but were the protagonists of that war. There was in that context a bipolarization of the world between two economic systems, US-led capitalism, and socialism, led by the USSR.

Although there are still Communist countries today, the capitalist system prevailed after the collapse of the Soviet Union in the early 1990s.