What caused the us and ussr to become enemies after being allies?

Hopefully you are talking about World War II. If you are then after Pearl Harbor, Japan was pretending they were strong. The USSR didn't want to get on their bad side so they switched teams. That is the main reason why we're not allies.

The U.S. got scared when the USSR created their first atomic bomb and they did not like that the USSR had helped Germany in the beginning of World War ll. Hope this helps you.