American Empire: Blood and Iron

As Turtledove's brilliant series The Great War came to its end, the United States of America, in alliance with Germany, had defeated Great Britain, France and the Confederate States of America in a bloody conflict known as the first World War.
As the 1920s begin, though, the seeds of a new conflict have alredy been sown. The United States, led by Theodore Roosevelt, swings wildly towards socialism. In Canada - now a US colony - nationalist terrorists strike against the new American oppressors. But it is in the Confederacy, trapped in a ruinous economic depression, where fascism begins to spread, and the fires are fanned by a charismatic leader who may again plunge the world into war.

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