If Nazi, Germany .......
Let's start the thread with... Had the Nazi not been defeated, or lasted longer, history was different, just how would our world have changed?
There are a lot of alternative-history science-fiction stories with that premise.
James Hogan's '
The Proteus Project' (written in the 1980's) begins with a US nuclear submarine returning from a cold war patrol in the present. But it quickly turns out that the adversary it faced wasn't the Soviet block, but Nazi Germany and fascist Japan astride all of Eurasia, Africa and South America. 'The Allies' were restricted to North America, with the boundary down in Panama or something. When Britain fell to the Germans in 1940-1, the King fled across the Atlantic to Canada and now resides in Ottawa.
The hot war between the Allies and Axis, and continued Axis expansion, eventually ended when both sides developed nuclear weapons, and a 'mutually-assured-destruction' stand-off resulted. But with the Axis holding the entire world, apart from North America, their advantage in natural resources and industry made it clear that the Allies' days were numbered. Their eventual defeat was inevitable.
So... the Americans developed time-travel, and hatched a plot to go back in time and give the secret of the atomic bomb to Albert Einstein and the other physicists, so that the Allies would have nukes first and would be able to nuke the Germans and Japanese.
But... things get a lot more complicated fast, since the time travelers, back in 1940, can't re-establish contact with the present. They are cut off back there. And then it appears that there are other time travelers lurking around too, sent back from a variety of different futures. And the story eventually moves from simple time-travel towards a many-worlds multiverse idea. Einstein helps them figure it out. Meanwhile, they have to convince Roosevelt and Churchill, while fighting off Nazi time-travelers and making sense of mysterious visitors from far stranger futures.