Pelgraff (The Yrden Chronicles Book 5‪)‬

Humans and the gorilla-like Pagayans lived in peace on the planet Pelgraff until the Damargs arrived. Now civil war has erupted.

Alan McLean, bitter ex-New Brittain police officer, accepts an offer to help train the men of an Interplanetary Brigade going there to fight. He finds himself drawn into the conflict even as politics makes the prospect of eventual victory increasingly unlikely.