Harry Turtledove 
Darkness Descending 
Book Two of The Darkness Series

In this sequel to the bestselling INTO THE DARKNESS, the country of Algarve is successful in its blitzkrieg tactics, but becomes bogged down in the desolate winter of Unkerlant, its main enemy. Algarve’s king decides to undertake blood magic, which amounts to genocide, in order to break the deadlock; when it is seen to work, its use elsewhere cannot be long coming. On a small island, a theoretical sorcerer may hold the only answer to this horror . . .Harry Turtledove has taken events similar to those from Earth’s wars and transplanted them to a wonderfully imagined fantasy world.