Howling Stars. Decker's War, #4

Every Marine has a story about someone he couldn't save, a face that would haunt him forever. When Command Sergeant Hal Tarra, one of Decker's oldest friends, vanishes on the Commonwealth frontier shortly after taking early retirement from the Corps, Zack knows what memories are driving the former Pathfinder, because he sees the same face in his worst nightmares. Naval intelligence isn't interested in resurrecting ancient history, especially the tale of a failed rescue after it had been consigned to the memory hole.
So Decker does the only thing he can, out of friendship and loyalty, and the hope that he might finally redeem the one they'd thought lost long ago: he orders himself on an unsanctioned mission, beyond Fleet support, risking everything for a friend. His intelligence colleagues never could understand the Marine promise, "No One Left Behind" but as he keeps reminding them, he's still one of the Few.