El Mercenario by Jerry Pournelle

Set in a future where Earth is part of a vast interstellar empire, the story follows John Christian Falkenberg, a seasoned military leader, as he navigates the complex political landscape of a war-torn planet. Tasked with quelling a rebellion, Falkenberg must employ his strategic prowess and combat experience to bring stability to a world on the brink of chaos. Amidst the backdrop of power struggles and shifting alliances, the narrative explores themes of duty, loyalty, and the moral complexities of warfare.