The American public has grown accustomed to the notion that very bad things happened in Vietnam—though for the entire ten year period, only one bad thing, My Lai, was accorded the label “atrocity.” The war in Korea, which the U.S. public has had difficulty knowing how to remember (and which historians designate “forgotten”), was—when thought about at all—seen as a coda to World War II rather than a prologue to Vietnam. It was a war more unpopular than Vietnam, but unlike Vietnam its unpopularity was forgotten except by the politicians who drew from it specific and limited lessons: do not lose a country; avoid Chinese intervention. And it ended where it began—South Korea saved for the free world, North Korea properly chastised, the Chinese contained within their borders. Not quite a victory, perhaps, but not a defeat, and as rapidly as could be, a war that in the main was attended to only by Americans who had relatives fighting there was filed away.

There were lingering issues—particularly the alleged collaboration of large numbers of American prisoners of war—but few wondered why the war had occurred or doubted its necessity. The only atrocities associated with it were attributed to North Koreans and Chinese troops. This is hardly unusual. As one expert in military law put it: “Battlefield war crimes committed by one’s own forces are almost never charged as such. Instead, they are simply alleged as the Uniform Code of Military Justice offenses of murder, rape or aggravated assault…. They are denominated war crimes only if committed by enemy nationals.” (No one at My Lai, for example, was charged with having committed a war crime.) And then one fall day in 1999, the New York Times published, on its front page, an Associated Press story about an event that had taken place under a railroad bridge near No Gun Ri in South Korea in July 1950.

The event was a massacre. Korean refugees, ordered out of their villages by American troops, were strafed by U.S. planes and then herded by U. S. soldiers under a railroad bridge and fired upon—in some accounts over a three-day period—as they huddled there for safety. The number of dead remains in dispute, though it is likely to have been several hundred and perhaps as many as four hundred. It seemed initially an inassimilable story, misplaced in the wrong war. All but historians of the Korean War were taken by surprise.

The same articles reporting what happened under the bridge incorporated in their accounts a set of extenuating circumstances. First, the civilians had given cause, because North Korean soldiers were known to infiltrate refugee columns in order to get behind U.S. lines. Second, the soldiers themselves did not really represent the U.S. military. They were untrained troops suddenly catapulted into combat from their comfortable occupation duty in Japan. Third, the circumstances were particularly fraught: U.S. troops were in chaotic re...