In Greek histories, Spartan mothers sent their sons to war with the commandment, "Come back with your shield, or on it."

Spartan mothers loved their babies, too -- they did not want to see dead bodies of their son brought back, as was the custom, sprawled on their shields. But if a warrior returned alive and unarmed it meant he had broken ranks and run. It meant he had thrown away the shield that protected -- not his own life, but, in the old method of fighting in phalanxes, the life of the man next to him. He had broken faith with his comrades; he had forgotten his warrior's code.

They wanted their sons back alive, but whole in spirit as well as body. They wanted them with honor intact. Everyone today who loves a soldier, sailor or Marine understand this. We want them alive, we want them victorious -- and we want them to have lives worth living when their battles are over.

Which is why we have to watch carefully, on many levels, the daily unfolding in Iraq. Roadside bombings and terrorist massacres make headlines, but incidents that miss the headlines can cut deeper. Like what might have happened outside the city of Samarra on the night of January 3.

Marwan and Zeidun, two Iraqi cousins, drove toward the city in a truck carrying ceramic floor tiles and toilets. They had broken down on the way, and they missed the curfew. An American patrol stopped them. It was the last patrol of the night. There are at least two versions of what happened next. And two things are certain: the truck they were driving was destroyed, and Zeidun is dead, drowned in the River Tigris.

None of this might have transpired, except that the cousins come from a prominent family, which made sure people knew about the case. It got into one of the excellent Iraqi blogs on the Internet. Now Slate.com has published a piece on the incident at Samarra. Slowly it is percolating into American awareness.

Marwan, who survived, says U.S. soldiers marched them to the edge of the water and pushed them in from a small bridge. The bridge is not high. The Slate reporter, who saw it, wrote, "Pushing someone in here would be like pushing someone into a swimming pool; there was no drop to speak of." But it was at the edge of a dam, and that means someone, especially a non-swimmer like Zaidun, could get sucked under. Perhaps that is what happened.

Col. Frederick Rudesheim is the 4th Infantry officer who has the unenviable job of overseeing operations in Samarra. "I have done my own investigation and reached my own conclusions," he said. After talking to Marwan, other witnesses, and the U.S. platoon, "I am absolutely satisfied that our soldiers did not do anything improper." Nonetheless, in the interest of having all doubts settled, he has ordered up an inquiry from the military's Criminal Investigation Division.

The platoon's story, that it dropped off the two Iraqis at the bridge and left them to walk back, is rather shaky. Why? Why there? And what happened to the truck? The Slate reporter came away with the theory that "The American patrol pushed the two cousins into the river -- without any murderous intent -- because they couldn't be bothered to detain them and then ran over their truck with a Bradley."

The CID investigation is underway, the results are pending. That is the truth about so much in Iraq today. But troubling incidents -- and the one at the dam in Samarra, if true, is deeply troubling -- can be a warning for the whole enterprise.

Modern armies sweep into their ranks hundreds of thousands of people. Not all are fit to be soldiers. Those who are not, when discovered, should be weeded out and sent home, and if they have committed crimes in the meanwhile they should be punished for them.

But this is not a matter of good soldiers and bad apples. Certain kinds of combat, or duty, wear down the military codes of honor. The warrior's code frays, then the seams fall apart. Then horrible things begin to happen.

Warrior codes, whether in Sparta or in West Point, distinguish soldiers from murderers. Warriors have rules that govern when and how they kill. Learning them is part of the purpose of military training. We give soldiers the power to take lives, but only certain lives, in certain ways, at certain times, and for certain reasons.

The purpose of a code "is to restrain warriors, for their own good as much as for the good of others," writes Shannon E. French, an assistant professor of philosophy and author of "The Code of the Warrior: Exploring Warrior Values Past and Present." "The essential element of a warrior's code is that it must set definite limits on what warriors can and cannot do if they want to continue to be regarded as warriors, not murderers or cowards. For the warrior who has such a code, certain actions remain unthinkable, even in the most dire or extreme circumstances."

Yet the danger of crossing that thin, sharp line that separates warriors from murderers is greatest in exactly the kind of conflict Americans face in Iraq: war not among great powers, evenly matched, but of well-equipped armies pitted against weak but merciless foes who hit and run and hide among civilians. Samarra is smack in the Sunni Triangle. It's the kind of place where graffiti reads "We will kill the Americans wherever they go!" It is the kind of place people blow up public buildings to make a political point. There is no warrior code in that; a terrorist is a terrorist, however he justifies himself.

But this is where the risk lies for the Americans. "Vietnam" has become an overworked cliche from the Left. Like "fascist," it's an important word from history, full of lessons, that has been drained of meaning by over-use. Back in the spring, every time the Coalition armies paused on the road to Baghdad and Basra to regroup, the vultures from the Left began to cackle about "another Vietnam." The rapid collapse of Saddam's military shut them up for a while. But now that the U.S. forces face an insurgent movement, they're at it again.

Ignore them; they're just parroting their cliches. But pay attention to Vietnam. It was the last time the U.S. got into a situation like this, and in parts of the military, the warrior code broke down, the door between soldiers and killers came unhinged, and a few good boys from America gunned down helpless peasant villagers.

If there was an act of brutality in Samarra, it should be punished without pity. It should be done publicly, for all to see. The troops need to see that the criminals in our ranks will be found and purged. So do the Iraqis. Otherwise, the hard work of winning hearts and minds in Iraq will be lost. Otherwise, the warrior code will weaken by that much more in the minds of American soldiers and Marines still trying to do an honest job.

It is not the justness, or lack of it, in a war that makes this happen. Japanese soldiers, brutalized by experience in China, did it to American soldiers in the Pacific and Americans did it in turn to the Japanese when they found out about it. Tennessee soldiers who fought with honor and discipline at Shiloh in 1862 turned into murderous bushwhackers by 1864. Many soldiers in Hitler's army behaved to the end with utmost military discipline. Some of the Soviet troops who defeated the Nazis raped and pillaged their path halfway across Europe.

When warriors and murderers clash, the murderers risk nothing but death. The warriors risk more. "Their only protection is their code of honor," French writes. "The professional military ethics that restrain warriors -- that keep them from targeting those who cannot fight back, from taking pleasure in killing, from striking harder than is necessary, and that encourage them to offer mercy to their defeated enemies and even to help rebuild their countries and communities -- are also their own protection against becoming what they abhor."