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.
the germans attacked the US, they were fighting & occupying US allies in europe. this impression of the nazis being benign or friendly toward US interests is completely false. britain and france still had outstanding debts from ww1, to say nothing of US capital investment threatened by nazi expansion.
so why did they engage? it was in their ruling class' self-interest, just like the first war. they would have done this regardless of how well or poorly the soviets were performing, but Stalin was sticking a boot in their ass to expand the front in the west the whole war