The bravest soldier becomes a coward when he follows an order to shoot unarmed people. The slaughter reported in this week's news of people on a Baghdad bus that didn't stop for soldiers at a roadblock is a case in point.

The rule in such a case is to shoot first and ask questions later. They call it a "rule of engagement" but there's no romance in it. We're talking about the engagement of hostile forces. Except that we're not. A bus driver who misinterprets a signal from foreign soldiers at a checkpoint can hardly be said to have engaged the soldiers, and that's what happened here. "Engagement" suggests a contact that both parties are aware of. An assault on an unwitting or escaping victim is not an engagement.

The rationale for killing innocents is that they might not be innocent. In the overwhelming number of cases, they turn out to be innocent, but they might not be, and so it's OK to kill them. For the men in suits who order the soldiers around, that's fine. No sleep is lost. But for the soldiers who have to do the killing, there's quick recognition that courage and valor are lacking in these "engagements," and the soldiers are hurt by this knowledge.

You can read their accounts on the Internet, in dozens of recent books, and in transcripts of the rare legal proceedings that have come on the heels of checkpoint killings. As the words of the soldiers so consistently tell us, there's nothing like a diminutive severed limb to take the excitement out of carrying a rifle. It's probably safe to assume that the images of slaughter will stay with them always and color the rest of their lives, especially when they cuddle their own kids.

These damaged young people put on a uniform to demonstrate their commitment and bravery and selflessness and they found themselves part of a brutal slaughter. Instead of valor, they were forced to acts of self-preservation against harmless women and children, acts so drenched in cowardice as to confer on them a badge of lifelong shame and dishonor. It shouldn't come as a surprise that young vets are killing themselves at an unprecedented rate.