Yeah, on the one hand, it seems reasonable to say that if Germany had pulled off a victorious peace in Europe, before America joined the fight, we wouldn't have cared much at all. Europe would have been left to sort itself out while we worried about Communism. I can remember reading an American children's book from the early '40s, from a very mainstream publisher, that described Hitler as a new Napoleon and had several flattering pictures of him in a friendly mood.
On the other hand, it seems clear in hindsight that powerful elements in America were spoiling to expand influence. Otherwise, why would we have cared about Communism as early as we did? Or been officially threatening Germany before they declared war? Not to mention our uncalled-for exploits in China and the Philippines (although that was long before Roosevelt). So I think if "Hitler had won" we might in fact have had a Cold War with Western Europe. It would depend on who -- fascism or communism -- became the biggest dog for us to fight. America was in the process of changing.