Great War #2: Walk in Hell book by Harry Turtledove

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 conflict continues and grows. Both presidents--Theodore Roosevelt of the Union and staunch Confederate Woodrow Wilson--are stubbornly determined to lead their nations to victory, at any cost. . .