Of course the US could have defeated Germany on its own. Germany was the second most powerful military in the world behind the US. You seem to forget that the US was fighting in two theaters at the same time (European and Pacific) thousands of miles apart from one another and across two gigantic oceans. Obviously, the Russians contributed massively in the European theater, as did the British empire. But it was the US economy that supplied them all. Meanwhile, the US was all but singlehandedly crushing the imperial Japanese in the Pacific theater. Can you name another power of the time that could do all those things at the same time? Not even close.