AFAIK, their war lay primarily in the Pacific, and beyond supporting the Brits and Russians materially, I’m not really sure why the US would want to involve themselves physically in the European theatre. I do feel fear of Germans beating them to the bomb might have something to do with it, but that’s just conjecture.
You have to take into account that to industrial capitalism, communism is not just a threat, it is THE existential threat.
The first Red Scare had already taken place, the US was scared of labour organization, which was at an all time high inter-war. The depression had just ended, returned soldiers from WWI had marched on Washington, Russian immigrants expelled on espionage charges, anarchists bombed wall street and harsh laws were put in place during and immediately after WWI to curtail the protection of free speech when invoked by dissidents.
This is only in the US, in Europe, South America and Asia communism was getting widespread public support. El Salvador, China, Finland, Bulgaria and Spain had all had communist uprisings or civil wars by the start of WWII.
The Soviets had also resisted the onslaught of Nazi Germany, the combined weight of all of continental Europe's industrial might pressed on them and they broke through, they had twice the number of men the US did. There would have been no illusion that the Soviets were not capable of taking all of Europe.
Both sides would have seen early that, if the Nazi's were beaten, the division of their territory would set the stage for the conflict between the US and the USSR to follow.