I'm no history buff, but I think that's unlikely.

It's true the US had an isolationist movement, and even business ties to Germany, but it would have eventually entered the war against Nazi Germany. After all, it was already supporting its allies (UK, Soviet Union, etc) via materiel through the Lend-Lease program, before the attack on Pearl Harbor ( http://en.wikipedia.org/wiki/Lend-lease )

Also, without wanting to diminish the participation of the US in the war, most of the effort was done by other countries, such as the Soviet Union.

Doubtless the final shape of Europe would have been different if the US had entered the war later, but Germany would have been defeated anyway.

What if the US stayed out of Europe and only declared war against Japan? If the US had invaded Manchuria while Stalin was still busy with Europe, the Chinese civil war might have panned out differently.

I think that unlikely. Germany would have fallen to the Russians. Soviet Europe would have stayed that way for a few decades before reverting to independent mixed economies. The real question is what would have happened between China and Japan, without the American nukes.