Question

Why did the united states and the soviets end up on opposite sides of the cold war?

Answer 1

Their relationship deteriorated over a rising level of tension created by distrust and differing ideas on how to rebuild Europe after the war. Prior to that, there was already a great deal of caution -- the West didn't quite trust the communists who wanted to spark a worldwide revolution, and the communists wanted to see the capitalists destroy each other in Europe.

When it was clear that the Soviets were more interested in extending their reach over Eastern Europe than in rebuilding it and eventually leaving, the friction only got worse sparking incidents like the Berlin Blockade.

The Soviets, on the other hand, felt they needed to position themselves in Eastern Europe in order to prevent future invasions from the West -- something that they were keen to avoid being the victim of again.

As the two continued to compete whether it was through intelligence, an arms race, or in making new friends around the world, a new war had begun...a Cold War between the two superpowers.

Answer 2

Also they were part of two competing systems, communism vs. capitalism, which you can add to the other response