Question

Why united states did not interfere in soviet union?

Answer 1

First of all, it should never be assumed that the Soviets were really our allies. Prior to the war the USSR was an international pariah. Most of the major countries in Western Europe and even the US and Japan had sent contingents to attack the USSR at it's establishment and since then the USSR had existed in a state of self imposed isolation that was reciprocated by pretty much every other country. It only came out of isolation in the late 1930's and only once it became apparent what a threat Nazi Germany would become. It attempted to coordinate with the Western Allies to contain the Threat but the West proved disinclined to work as closely as the Soviets hoped. With the west determined to maintain it's policy of appeasement the Soviets decided the best policy was to come to terms with the Germans, leading to the Nazi-Soviet Pact.

The Soviets were pleased to have this agreement but that soon changed when the Germans overran France in just 6 weeks, the Soviets had counted on the Germans having a long war with France as they had had in the First World War. With the French now defeated the Soviets feared the Germans would attack them next which they ultimately did. It was this more than anything else that made the Soviets a member of the Allies, not a genuine desire to fight the Germans. 

When the war ended the Soviets were concerned about preventing anything like the German invasion from happening ever again. To the Soviets this meant grabbing territories in Eastern Europe and create a buffer of Friendly communist states. The Western Allies had wanted these countries to be liberated, not made into satellites an felt betrayed by this Soviet self-interested move. This sense of betrayal combined with evidence that the Soviets had been actively spying on their allies created the a lot of the distrust between the allies. This was worsened by the fact that these countries had competing ideologies.

This is by no means an exhaustive answer but it serves as an outline. This is a question that whole book can and have been written about but these are the basic facts about the origins of the Cold War. Feel free to pose questions or add comments to help create a more exhaustive answer