Captain's Fury: Jim Butcher

A compulsively fast-paced fantasy adventure, set in a world where courage and ingenuity vie with magic and power.
Tavi of Calderon, now captain of his own Legion, has been fighting a bitter war for two years. Then he discovers the invading Canim warriors are harbingers of a far greater threat. The Canim are being hunted in their turn by a savage race that forced them from their homeland - and which has pursued them to the Aleran borders. With options fast running out, Tavi proposes an alliance with the Canim.
But the Senate s new military commander wishes only to wipe out the Canim scourge , and would also kill Aleran slaves that have sought freedom with these aggressors.
Tavi must reconcile Aleran and Canim, slavemaster and slave, Citizen and Proletarian, if an alliance is to be forced. And he must lead his Legion in defiance of the law, against both friend and enemy - before the greatest army of all launches its assault.