Great war (Harry Turtledove)

From the master of alternate history comes an epic of the second Civil War. It was an epoch of glory and success, of disaster and despair. . . .
1881: A generation after the South won the Civil War, America writhed once more in the bloody throes of battle. Furious over the annexation of key Mexican territory, the United States declared total war against the Confederate States of America in 1881. 
But this was a new kind of war, fought...

The year is 1915, and the world is convulsing. Though the Confederacy has defeated its northern enemy twice, this time the United States has allied with the Kaiser. In the South, the freed slaves, fueled by Marxist rhetoric and the bitterness of a racist nation, take up the weapons of the Red rebellion. Despite these advantages, the United States remains pinned between Canada and the Confederate States of America, so the bloody confilct continues...

After the Great War had ended, Americans find themselves trying to rebuild their lives, and their country, but threats from Canada and Africa may tear their dreams apart.