Can’t help but to wonder how history changes if we wouldn’t have entered either war. Especially the 2nd. I think the Nazis would have prevailed, or at least survived on, controlling much of Europe if we hadn’t intervened. Without the war on two fronts, Germany might have held on to much of Russia. If they would have taken Russia completely, our ‘Cold War’ wouldn’t have happened. Who knows what the world would have looked like then. The last half of the 20th Century would have surely looked very different.
If we hadn’t entered the Great War, Germany would have won, there would have been no Treaty of Versailles, no German reparations, and no ax for disgruntled Austrian painters to grind.
Ignoring all the above, had Germany not declared war on the US after Pearl Harbor and we had managed to stay out of the European component of that war, the Germans would probably have conquered Russia to the Urals, and it would have settled out there.
That’s my guess.
If we had stayed out of the 1st, Germany likely would have won or fought to a standstill. There would have been a treaty with some face-saving provisions, and territorial or colonial swaps between the sides, and life would have gone on.
The upshot would have been no WW2 as we knew it. Maybe a war between the west and Japan or some colonial war in Africa.