NEW HAVEN — In the summer of 1968, John Shepherd Jr. enlisted in the Army, figuring that the draft would get him anyway. By January 1969, he was in the Mekong Delta, fighting with the Ninth Infantry Division.

Within a month, his patrol was ambushed, and Mr. Shepherd responded by tossing a hand grenade into a bunker that killed several enemy soldiers. The Army awarded him a Bronze Star with a valor device, one of its highest decorations.

Yet the medal did little to assuage Mr. Shepherd’s sense of anxiousness and futility about the war. A few weeks after his act of heroism, he said, his platoon leader was killed by a sniper as he tried to help Mr. Shepherd out of a canal. It was a breaking point: his behavior became erratic, and at some point he simply refused to go on patrol....