Harry Turtledove is the name of an author of alternate history and fantasy novels, perhaps being the most well-known author in the former genre.
Turtledove graduated from UCLA with a doctorate in Byzantine history; several of his fiction works have reflected on that, including his Agent of Byzantium stories and his Videssos fantasy world. He also wrote a historical fiction novel on the reign of emperor Justinian II.
However, Turtledove is best-known for his two alternate history series Worldwar and Southern Victory (TL-191). Worldwar (and its follow-up series Colonization) dealt with an alien invasion of Earth during World War II, and humanity's subsequent dealings with a half-conquered Earth during the Cold War. Southern Victory focused on the United States and the Confederate States following a Confederate victory in the Civil War, with occasional forays into Canada and Mexico. The series covers the history of this alternate North America through WWI, the rise of totalitarianism, and WWII.
Many alternate history fans have criticized Turtledove for leaning too closely on the parallels with OTL; others, however, enjoy his work and applaud him for making alternate history a more mainstream sci-fi genre.