The basic idea is that, after the Battle of Gettysburg, the South’s commanding General Robert E. Lee is visited by someone who introduces him to the AK-47 assault rifle. As you might imagine, the story takes off from there!
An Amazon reviewer writes:
“On one level Harry Turtledove’s “The Guns of the South” reminds me of the Saturday Night Live skit which asked the question, “What if Napoleon had a B-52 at the Battle of Waterloo?” This alternative history novel about the Civil War asks “What if the South had AK-47s during the Civil War?” The answer is, of course, they would have won their independence. But ultimately “The Guns of the South” is more about what type of nation the Confederacy would have become.”