Jaws of darkness - Harry Turtledove

The grand conflict for control of the continent of Derlavai rages on, in a battle with all the drama and terror of the Second World War - but for artillery and bombers, read magical fire and dragons. Yet hope may be dawning at last. The terrible onslaught of the conquering forces of Algarve - who power their battle magics with the life energy of their murdered victims - begins to founder as it runs into Habakkuk: a sorcerous ship of ice used by the embattled nations of Lagoas and Kuusamo to ferry their deadly dragons across the seas to war. But though the tide has begun to turn, the conflict is far from over. The widely disdained Kaunians still struggle desperately to escape as the Algarvians kill them by the thousands - for life energy, but also simply for the crime of being Kaunian. And as the death of innocent civilians on both sides continues to feed the flames of war, those who have struggled to survive and preserve their freedom have only their will to see them through.