If Germany defeated Russia, they would have seized vast resources and conscripted possibly millions more soldiers that would have seriously bolstered their efforts to conquer Europe.

The US was never "pro communism" and the alliance with the Soviets was tenuous at best and was only possible due to each other's need to defeat a common enemy. Roosevelt condemned many actions of the Soviets leading up to and during the war, but knew that overall Nazi Germany was the greater threat to the free world at the time.

Same with Japan... If Japan had conquered more areas of China, the resources they desperately lacked would have been secured and could have changed the outcome of the Pacific War. A major factor that led to Japan's defeat was their lack of resources.

You could start learning more about the US-Soviet Alliance here:

https://history.state.gov/milestones/1937-1945/us-soviet

It's a short light read that answers your question very directly. The alliance was not desirable, but it was necessary. There was hope for great cooperation afterward but without the common enemy of the Nazis, the previous tensions between the nations persisted and the Cold War was born.