Where the story takes place in a setting that differs from our current history, this is said to be an Alternate History.
The difference needs to be something fairly major, not something that is AH because of sloppy research :-) or the simple passage of time. The change that is being proposed should also be somewhat believable - Hitler suddenly nuking the Allied forces in 1945 would not be! Also not really acceptable are stories set in what is now the present that show this present as different from what has really happened due to unexpected developments - many science fiction books were written at the height of the Cold War and at their most positive foresaw a continuing stand-off between the Soviet Union and the West.
The main themes of AHs tend to have the American Civil War and WW2 as the Point of Departure (PoD). This is not too surprising as most writers of AHs are American. Where the Civil War is concerned, the Confederacy generally gains its freedom, whilst the Axis Powers winning is the main change in WW2 AHs. Another favourite PoD is the American Revolutionary War where the rebellion is crushed.
The leading proponent of AHs at the moment is Harry Turtledove. As well as writing books having all three PoDs, Dr Turtledove has also written a series of short stories working out some of the possibilities if the East Roman Empire had not fallen.
See also:
Return to the sub index