They played the role of an allied power but were still communist so the U.S always had an eye on them.the reason for the partnership to collapse is because of the Russian was trying to take over all of Europe as they took countries back from Germany and making them satellite countries and then we had to start the cold war to get them all freed where the Russia / USSR at the time said that they had A-Bombs just like us so we had to do something and thats when the cold war started. the main goals of the USSR were to take over all the destroyed countries and all of Europe with there satellite countries and themselves but the US's Marshall Plan stopped them form doing so.
A common explanation is that the Civil War was fought over the moral issue of slavery. In fact, it was the economics of slavery and political control of that system that was central to the conflict.
