Drive to the East — Harry Turtledove

It's 1942. For twenty-five years, the USA and the CSA have been entrenched in an era of simmering hatred, locked in a tangle of blood-soaked battle lines, modern weaponry, desperate strategies, and the kind of violence that only the damned could conjure up for themselves and their enemies. In Richmond, Confederate president and dictator Jake Featherston is shocked by what his own aircraft have done in Philadelphia -- killing U.S. president Al Smith in a barrage of bombs. Featherston presses ahead with a secret plan carried out on the dusty plains of Texas, where a so-called detention camp hides a far more evil purpose. As the untested U.S. vice president takes over for Smith, the United States face a furious thrust by the Confederate army, pressing inexorably into Pennsylvania. But with the industrial heartland under siege, Canada in revolt, and U.S. naval ships fighting against the Japanese in the Sandwich Islands, the most dangerous place in the world may be overlooked.