World War II was a very pyrrhic victory for the Allies. If you look at the numbers, you'd actually think Germany was the one that won the war.
America was almost as exhausted of the war as the other countries, though they had the advantage of being unscathed at their home-ground while most of the rest of the world was in ruin. The Russians would have been a problem. A big one for sure, they fight until there's none left. Ask Napoleon (or Hitler though he could have won if he was less stubborn, the world dodged a bullet there).
If the UK and it's Empire sided with them, we'd have World War 3 right after 2, which wouldn't have done the human race any favors. If nuclear warfare continued, it would be bombing after bombing on both sides. The UK knew how to build such a thing but hadn't actually done it, the Russians were close and even Germany itself was just a stone-throw away from developing a bomb. The Nazi's advanced weapon technology and knowledge was up for grabs. Americans already in Europe would also have been surrounded in that case, so that army would be gone.
A lot of things could have happened if any one power would have decided to continue the war or start a new one, but let's just say the world would have been in even deeper sh*t.
