Harry Turtledove 
How Few Remain 

A generation after the South won the first American civil War, America writhes once more in the bloody throes of battle. Furious over the annexation of key Mexican territory, the United States declares total war against the Confederate States. And so, in 1881, the fragile peace is shattered.But this is a new kind of war, fought on a lawless frontier where the Blue and the Grey battle not only each other but the Apache, the outlaw – and even the British Redcoat. For along with France, Britain enters the fray on the Confederate side. 'The wizard of If.’ Chicago Sun-Times’The standard-bearer for alternate history.’ USA Today