Back in the 1960s, I criticized the Vietnam War, in part, by contrasting it with the Korean War. This was a way of distinguishing myself, and the part of the antiwar movement to which I belonged, from those leftists who insisted that no American war could possibly be just. It was also a way of arguing that Stalinist aggression sometimes required a military as well as a political response. I still believe that the massive invasion of the South by the North Korean army should have been resisted—even if, as we have now learned, the forces that led the resistance committed criminal acts. But it is important to acknowledge the crimes and examine their causes.

Some of the causes are easy, even comforting, to list, and so they were immediately evoked by people eager to defend, if not quite to justify, the killings described in the above AP dispatch. The American soldiers involved in the massacre were raw recruits, rushed into combat from Japan, and they were in headlong retreat. These are circumstances that, in many wars, have made for both rage and panic, emotions more easily directed against the civilian population than the attacking army. Frightened and inexperienced soldiers are more likely to kill both brutally and randomly; military discipline reduces civilian casualties. But this can be, at best, only a partial explanation of what happened at No Gun Ri. For the killings there were not committed “in the heat of battle.” They seem to have been the direct result of orders issued from U.S. Army headquarters, and the officers giving the orders were surely not raw recruits. They were veterans of World War II, which ended only five years before the killings at the bridge, and which was followed by a major effort, at Nuremberg and Tokyo, to teach future generations a lesson about the crimes of war. No Gun Ri is a case of refusing to remember the lesson.

The trials had barely ended in 1950, but it is now clear that American officers had not acquired that heightened awareness of noncombatant immunity that Nuremberg and Tokyo should have taught—nor had they incorporated this awareness into the training of new soldiers. It couldn’t have been possible to study accounts of the Nuremberg trials and then issue the command that came from the First Cavalry Division: “No refugees to cross the front line. Fire [at] everyone trying to cross lines. Use discretion in case of women and children.” It couldn’t have been possible to undergo a training in military ethics and then obey commands of that kind. Discretion is just what soldiers don’t have when they confront civilians.

Clearly, American officers, coached perhaps by their Korean counterparts, believed these to be hostile civilians: “all civilians seen in this area are to be considered as enemy and action taken accordingly.” That same perception, spread across the whole of the civilian population, was critical in making Vietnam an unjust war. You can’t fight in def...