The thesis of the book is that in 1917, German victory was inevitable due to the inevitability of total French collapse and the British running out of ammunition. But American entry allowed a vast influx of arms, munitions and manpower which turned the tide decisively enough to make the war last long enough for the German public to be starved into revolution.

So, the story picks up in 1881, when the South annexes Chihuahua and Sonora from Mexico and the US declares war. The US loses and goes socialist.

This sets up the US as a Russia-analogue and the CSA as a Germany-analogue.

Then there's a series with an alternate-WWI, where the CSA/France/Britain/Russia loses to the US/Germany/Austria-Hungary.

And then radical groups go Nazi in the CSA and holocaust on blacks and the alternate-WWII series happens.

Harry Turtledove isn't the greatest writer in the world...he can't write sex scenes worth a damn yet insists on having a dozen or so in each book...but the alternate history is fun.