Fox and Empire — Harry Turtledove

THE RELUCTANT EMPEROR

Ever since the devastating Werenight that spelled the end of the Elabonian Empire, Aragis the Archer has been saying he doesn't want war with Gerin the Fox, also called King of the North. But the Archer is ambitious and feels that his time is slipping by. So he makes one tiny little threat that turns out to have not such tiny consequences. Because the Fox must respond to this threat, and the Archer must again riposte.

Then, just as things are getting "interesting," the Empire knocks with great authority on the door to the Northlands. "Submit or die" is the message, and suddenly the Archer and the King of the North are allies once again. . . .