Americans have a dysfunctional relationship with 21st-century wars. Most of us do not fight in them, see the carnage or live with their physical and psychological ruins. Yet we cannot heal our own nation unless we reckon with their monstrous futility.

For Graham Platner, that reckoning began when he was a 20-year-old infantryman in Iraq. His company was constructing a patrol base near Falluja. To build it, they hired locals who often brought their kids to the work site. One day, a mortar round fired by insurgents landed where they were congregated. There can be no more senseless death than losing a child, a reality Mr. Platner had to confront as he administered first aid and then encountered distraught parents at a casualty collection point. He still remembers the sight, smell and feel of those lost children, as well as the anguish in their parents’ eyes.

Mr. Platner recalled this experience to me as he drove his truck through Maine, campaigning for the Democratic nomination for Senate. Last fall, his candidacy was rocked by revelations from his past. As a young Marine, he got a skull and bones tattoo on his chest that resembled a Nazi symbol (he denied knowing its meaning). His history on Reddit includes offensive comments that he attributes to a long process of dealing with the trauma of war. Yet while Democratic insiders in Washington were prepared to write him off, today he packs town halls, and polls show him leading Gov. Janet Mills of Maine by around 30 points.

One reason for this success is that Mr. Platner sounds radically honest by the standards of American politics, including when he talks about his own service. “There’s this thing I often think about,” he said, recalling the incident near Falluja. “Those kids were killed because we were spending money to build this base that probably doesn’t exist anymore.” His voice slipped into the present tense as he put himself back into the moment when the mothers arrived to pick up the remains, his own life in front of him like a storm cloud. “How horrifically wasteful this is.”