Mr. Hornfischer is the author of three works of World War II history: "The Last Stand of the Tin Can Sailors," "Ship of Ghosts" and, most recently, "Neptune's Inferno: The U.S. Navy at Guadalcanal."

Bob Hagen knew the worst of battle while serving on destroyers in the Pacific during World War II. He saw action at Guadalcanal. He was the gunnery officer on the USS Johnston when it was hit hard in the Battle Off Samar near the Philippines on October 25, 1944. For two hours he directed the ship's main guns, firing gamely at an overwhelming enemy. A Japanese shell turned two officers standing on the flying bridge, 10 feet below his station, into a pink mist. When the order to abandon ship came across, Hagen found himself floating in shark-infested waters watching the Johnston sink. His best friend, the ship's doctor, Robert Browne, was still aboard, refusing to seek safety until all the wounded had been evacuated. Hagen saw Browne re-entering the wardroom when a large shell from a Japanese warship followed him inside. At that moment, the war crystallized as a hard-to-discuss horror.

Hagen was a hard man, and proud. Even 60 years out, he was still a bit curmudgeonly talking about the dramatic naval history he had been part of. But recounting Browne's death to me in 2001, a man who had fought heroically in a suicidal defense of a small U.S. carrier task unit supporting the invasion of Leyte could only swallow back his sadness. That look in his eye and break in his voice took me past the veneer of his ever-ready chagrin and bravado. They took me to the things that have never left him.

One of the last surviving officers of his martyred warship, Hagen had invited me to his San Antonio condominium for an in-depth interview. I was a young historian writing a book about the Battle Off Samar, and Hagen's conversation was my entrée into the USS Johnston's wardroom. Knowing him enabled me to give a touch of life to the legend of men such as his commander, Ernest Evans, whose exploits in the battle earned him the Medal of Honor. In Hagen's presence I felt the weight of history and of a desperate naval battle the likes of which would never take place again.

Bob Hagen died in 2009, and the rest of his generation is soon going where we all must go. About 1.8 million World War II veterans remain alive today. That's less than half the number of 2003. When these voices go silent, those of us who write about the war will lose the benefit of living engagement. We will work as our Civil War colleagues do: from documents and recordings and nothing else. What will be gone when these are the sole primary sources is not the facts themselves but the spark that can bring them to life. Diaries and oral history transcripts can let us know a man's thoughts and deeds. But truth is also revealed through tone, emotion and context—and it can be plumbed responsively in real time to discover what was most important.

For those of us who have never served in uniform, it's easy to see World War II as a grand, sweeping drama, featuring actors large and small driven by a sense of overriding mission, all sins and failings vindicated by victory. Yet for the veterans I meet, the war is often about something else entirely. Any talk of it brings them back to a single, pervasive memory sequence: a moment of impossible decision or helplessness when, through their action or inaction, they believe, a comrade paid the eternal price. They can't talk about the war without reliving their powerlessness to influence its predations, without revealing how it changed them....