Walk in hell - Harry Turtledove

It's 1915, the Great War is intensifying, and the time of darkness has come. The slaves of the south have risen against their masters, taken on the creed of Bolshevism and are attacking the Confederacy from within. But the United States remains pinned between their weakened southern rival, and their other bitter enemy, Canada. But both Presidents - Theodore Roosevelt of the Union and staunch Confederate Woodrow Wilson - are stubbornly determined to lead their nations to victory, at any cost.

Meanwhile the new and poisonous weapons of tanks, gas and planes are starting to make their presence felt at the front. It's total war for the first time in human history and it's ordinary people on both sides who are the ones to begin suffer...

'The wizard of If.' Chicago Sun-Times
'The standard-bearer for alternate history.'
USA Today