Question

What event lead to the former alliance between the Soviet Union and the United States to unravel?

Answer 1

The differences in how the war affected each nation, with the US having less destruction and casualties compared to the Soviet Union is your best answer.

Another reason is the fact that they used two seperate types of government, and each tries to impose it's own kind on other countries in an effort to be able to be the "leader of the world".

hope this helps