An excellent science fiction/fantasy writer who is know as "The Master of Alternate History". Has several full series based around the major wars of the century, including "The Great War" series, which starts with the novel How Few Remain, a story set just post a civil war in which the north lost. He is best known for the novel Guns of the South, another novel in which the south wins the civil war...thanks to the intervention of some time travelers.
Some other series he's done include "WorldWar", in which the Second World War is interrupted by the interference of alien invaders, and the "Videssos Cycle", a story centered around an alternate Byzantium. He's also done several novels like Guns of the South, which site outside any series. Most of these are novels filled with excellent short stories.