Alternate History

From Turtledove

Alternate history is a subgenre of historical fiction (and sometimes of science fiction). It involves the changing of a certain historical event to produce a different outcome and traces the consequences of that alteration through historical events effected by it, comparing and contrasting them to their OTL equivalents.

Many of Harry Turtledove's best known and most popular works fall into this category, including the Southern Victory series and the Worldwar series.