Germany in 1933. Hitler takes power. Things go as they did, historically---except that Hitler does not scapegoat and persecute the Jews. He also defers to his generals and advisors.
Einstein does not leave the country. Honored and rewarded for his work, he helps Germany develop atomic energy, which other scientists apply to making an atomic bomb.
Listening to his armaments minister, his generals, and Goering, Hitler does not start a war until Germany has a huge technological advantage over its rivals. When he does decide to invade Poland in 1945, Germany has:
1. Nuclear weapons.
2. Ballistic missiles.
3. Cruise missiles.
4. Surface-to-air guided missiles.
5. Jet aircraft.
6. The most advanced tanks in the world.
7. Nuclear powerplants.
8. An advanced highway system and a highly mechanized army.
9. The world's best pretzels.
France and England are quickly overrun. Russia sues for peace after Moscow and Leningrad are nuked. Hitler takes his time and builds a massive invasion fleet, and the US lasts about a year before it is destroyed.
We came very, very close to something like this happening. It's sobering to think how close Germany came to crushing everybody as it was. Hitler probably couldn't have conquered North America at first, but eventually...