Guns of the South by Harry Turtledove (1992)
In this alternate history, General Robert E. Lee finds that he is able to win the Civil War, as a mysterious group of men with questionable motives provide the Confederacy with a weapon the world of the 1860s has not yet seen: the AK-47.
Though the founding premise of the book is far-fetched, you’ll need to suspend your disbelief no further. The book is incredibly-well researched, and captures the gritty feel of the era and the personalities of its characters in rich detail, from the attitudes of a defeated Abraham Lincoln to the opinions of the more progressively-minded sergeant-turned-schoolteacher Nathan Caudell. I think it’d thoroughly please a reader of traditional historical fiction as well as any harder military, political or sci-fi fan.