The Marines, including the newly christened "Sledgehammer," continue the battle of Peleliu against an enemy determined to fight to the last man. Devastated by the loss of a revered leader, and witnessing unimaginable barbarity on both sides, Sledge veers to the very edge of moral collapse. Their objective finally secured, the Marines return to Pavuvu fundamentally changed by their experience on Peleliu.

After battling across the island of Okinawa for more than a month, Sledge and the rest of the 1st Marine Division are ordered to relieve an Army division that has been in combat against the most strongly defended Japanese position on the island. The primordial conditions and the moral dilemma posed by the presence of civilians put tremendous strain on the physical and psychological endurance of Sledge and the other Marines.