Debt's Pledge. The Void Eternal, #1

He was a soldier without a war, alone and unwanted, until he discovered an alien enemy... Jefferson Odett found a skull on a blasted world on the edge of human space, just before the campaign against the rebel stationers was called off. A skull that was not human. When the transport taking him home is attacked by a strange alien vessel, crewed by fearsome creatures, Odett barely manages to escape with his life.
Convinced the skull he found was from the same species as the aliens who attacked his transport, Odett believes they represent a deadly threat to humanity. The commanders of Earth's military disagree and dismiss his claim. They send him to a remote outpost of a few hundred colonists. Saddled with a platoon of undisciplined and mostly green soldiers, Odett finds himself in an unenviable command on a backwater world.
But there is something else on this forgotten world -- something frightening that he has encountered before. Odett and his platoon of green soldiers, hard cases and discipline problems are all that stand between the colonists and an alien army of unspeakable power who will kill any species that gets in their way. Jack Mackenzie delivers an action-packed Science Fiction adventure with plenty of heart, heroes, danger and excitement.