We had been on the island for about an hour when we found the first skeleton. It was a pile of yellow bones tucked inside a cardboard box. Mark Noah squatted down for a look. He is a stocky man of 48, with a light buzz of blond hair and the wind-beaten eyes of a lifelong outdoorsman. Since 2008, he has been traveling to the tiny Pacific atoll of Tarawa to search for the remains of more than 500 Marines who died there in World War II. Sometimes locals dig up their bones and leave them in his storage locker.

Noah reached into the box and pushed aside a fragment of cranium to remove a curved metal plate. “Wow,” he muttered. “Clearly a World War II burial with the helmet.” He passed it to the man crouching next to him, Bill Belcher, and added, “It looks American.”

Belcher nodded. “That’s what I thought when I saw it.” He laid the piece back in the box and picked up two sections of jawbone with the teeth still attached. They fit together into a complete lower mandible, which Belcher held close to his glasses, squinting. Noah pulled another hunk of metal from the box. “And this is a hand grenade,” he said. He shook his head and smiled. It was all pretty normal on Tarawa.

This week marks the 70th anniversary of the Battle of Tarawa. It was a fight that lasted only three days, but they were among the bloodiest in 20th-century American history. By the time the battle ended, more than 1,100 U.S. Marines lay dead on the sandy earth and churning water....