Hammer and Anvil — Harry Turtledove

THE RED BOOTS
Videssos was beset by enemies. A pretender held the throne--a despot who cared little that barbarian hordes and rival realms carved away at his empire, so long as the wealth and booty of the land satisfied his unbridled appetites.
Few stood against him. And those few soon found their heads on pikes.
Only one name held hope for freedom: Maniakes. And from his exile on the very edge of the civilized world, young Maniakes took up the challenge, rallied his forces, and sailed off to topple the tyrant.
But the tyrant would use every means at his disposal--fair or most hideously foul--to destroy the crusading upstart. And even if Maniakes could stay alive, he would still have to pull together a battered, divided land as well as fend off a host of enemies--and thwart the former friend who had become his empire's most deadly
foe!