The Great War, book 4 : Breakthroughs
Harry Turtledove2001

Is it the war to end all wars--or war without end? What began as a conflict in Europe, when Germany unleashed a lightning assault on its enemies, soon spreads to North America, as a long-simmering hatred between two independent nations explodes in bloody combat. Twice in fifty years the Confederate States of America had humiliated their northern neighbor. Now revenge may at last be at hand.

Into this vast, seething cauldron plunges a new generation of weaponry changing the shape of war and the balance of power. While the Confederate States are distracted by an insurgency of African Americans who dream of establishing their own socialist republic, the United States are free to bring their military and industrial might directly to bear--and to unleash the most horrific armored assault the world has ever seen. Victory is at hand. But at a price that may be worse than war itself . . .