The first volume of the American Empire trilogy from Harry Turtledove,
"The Wizard of If".

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.

Now as the 1920s begin, though, the seeds of a new conflict have already
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.