Into The Darkness

When the Duke of Bari suddenly dies, the neighboring nation of Algarve, long seething over its defeat a generation ago in the Six Years' War, sees its chance to bring Bari into the fold...an action which the other countries surrounding Algarve cannot, by treaty, tolerate. As nation after nation declares war, a chain of treaties are invoked, ultimately bringing almost all the Powers of Derlavai into a war of unprecedented destructiveness.
For modern magic is deadlier than in ears past. Trained flocks of dragons rain explosive fire down on defenseless cities. Massed infantry race from place to place along a network of ley-lines. Rival powers harness sea leviathans to help sabotage one another's ships. The lights are going out all across Derlavai, and will not come back on in this lifetime.
Against this tapestry Harry Turtledove tells the story of an enormous cast of characters: soldiers and generals, washerwomen and scholars, peasants and diplomats. For all the world, highborn and low, is being plunged by world war...into the darkness.