Any time you propose re-resolving a historical situation with new possibilities, many different things could happen. However, my general guesses, based on having read a few histories, and played several simulation games, is:
WW1: The war would have lasted longer, perhaps long enough for the influenza epidemic to hit, which killed more people than the war did, and so I imagine would reduce everyone’s will to fight even further, and result in a peace treaty. Eventually there would have been an armistice, probably still going the way it did, but perhaps not as soon or as strongly negotiated. If the treaty terms were more favorable to the Germans, Hitler would have had less resentment with which to fuel support for his platform, and WW2 might not have occurred.
WW2: Without the US in the war, Japan would have dominated China and be in a position to start campaigning into the eastern USSR and possibly intervene in other places as well. That could depend on whether the US was still a potential check or not. Germany would not have had nearly as much to worry about from Britain in Europe and Africa without the Americans, which could have allowed them to engage the USSR differently. With no US Lend Lease support for the USSR, their army would have had less equipment. I think a German defeat of the USSR would have been possible but by no means certain.