For insurgents, there is no more powerful propaganda tool than the claim that their adversaries are employing force in an indiscriminate manner. It is even better for the insurgents’ cause if they can credibly charge the forces of the counterinsurgency with the targeted killing of noncombatants. For many people even today, the entire Americans enterprise in Vietnam is discredited by the belief that the U.S. military committed atrocities and war crimes on a regular basis and as a matter of official policy. But as Jim Webb has noted, stories of atrocious conduct, e.g. My Lai, "represented not the typical experience of the American soldier, but its ugly extreme."

In the quest for its own My Lai, the anti-Iraq war faction in this country has had to settle for Abu Ghraib, by far the most hyped stories of the war. But now, allegations of multiple murders in the town of Haditha, an insurgent stronghold in al Anbar Province, may provide them with the incident they need. According to published reports, a number of Marines from the storied 3rd Battalion, 1^st Marines, 1^st Marine Division are accused killing more than 20 Iraqi civilians in retaliation for the death of one of their comrades by a roadside bomb in November 2005.

The Marine Corps originally claimed that the Iraqis were killed by an insurgent bomb or during a firefight. But in response to allegations by Time magazine, the Naval Criminal Investigative Service (NCIS) began an investigation of the Haditha incident. A separate administrative investigation by Army Maj. Gen. A. Eldon Bargewell should be delivered soon to Lt. Gen. Peter Chiarelli, the operational commander of the multi-national force in Iraq, to determine whether there was an attempt to cover up the incident.

It is important to note that the investigation is still incomplete but that hasn’t stopped opponents of the war from using the incident in Haditha to advance their agenda. Last Wednesday, Rep. John Murtha, (D-PA), a vociferous critic of the war, broke the story, claiming that Marines in Haditha had "killed innocent civilians in cold blood." This incident, said Murtha, "shows the tremendous pressure that these guys are under every day when they’re out in combat." Appearing Sunday on This Week on ABC, Murtha went farther, claiming that the shootings in Haditha had been covered up. "Who covered it up, why did they cover it up, why did they wait so long? We don’t know how far it goes. It goes right up the chain of command."

Murtha’s attempt to use the Haditha incident for his own political purposes should be obvious to everyone. But if his description of the event—a cold-blooded killing of innocent civilians—is true, then those Marines committed a bona fide war crime. What, if anything, can be said in mitigation?

Atrocities and war crimes are acts of violence in wartime the brutality and cruelty of which exceed military necessity. They include, but are not limited to, looting, torture, rape, massacre, mutilation of the enemy dead, and the killing of captured soldiers or noncombatants.

The West has placed three constraints on its conduct warfare: proportion, discrimination, and the positive law of war. Proportion means that particular actions must be proportionate to legitimate military necessity and not involve needless suffering or destruction. Discrimination means that direct intentional attacks on noncombatants and non-military targets are prohibited. The incident at Haditha appears to be an example of this last category.

If civilians in Haditha were killed in revenge for the IED attack, the action violated the principle of discrimination and the positive law of war, which derives from conventions, customs, the general principles of law, decisions in international law, and the writings of authorities. Standards regulating the conduct of war have followed two general paths: "Geneva law," protecting victims and innocents; and "Hague law," regulating land combat.

The law of war attempts, insofar as it is possible, to civilize war. The law of war seeks to strike a balance among the principles of military necessity, humanity and chivalry and to employ the public conscience of civilized nations to restrain war. The positive law of war thus attempts to codify the principle that belligerents do not have an unlimited right to harm their adversaries.

The key to applying the law of war to particular situations is the principle of military necessity. This principle holds that subject to the principles of humanity and chivalry, a belligerent is justified in applying the amount of force necessary to achieve the complete submission of the enemy as soon as possible, with the least expenditure of time, life, and resources.

Military necessity recognizes that a commander’s overriding concern is the accomplishment of his mission and the safety of his troops. One would not attack a populated area, increasing the risks of civilian deaths, unless such attack was essential to the campaign. Humanity is the self-evident recognition of the fact that one’s enemy is also a human being. Prohibitions against killing or torturing prisoners, or the generally recognized obligation to provide medical treatment to wounded prisoners, flow from this principle. Chivalry is the customary recognition of the idea that the strong protect the weak. Soldiers do not declare war on women or children because it is dishonorable to do so. If women or children engage in war, however, the principle of military necessity usually takes precedence over chivalry, but if the facts are as described by Murtha, military necessity was not a consideration in this case.

Nonetheless, we still don’t have access to all of the information some of which could absolve the Marines under investigation of a war crime. As Tom Ricks has reported in the Washington Post, individuals familiar with the investigation have indicated that message traffic and video from an unmanned drone may affect the outcome of the investigation.

Haditha has all the makings of a terrible story. But I would say of it what I’ve said of My Lai: It was an extreme case. Anyone who has been in combat understands the thin line between permissible acts and atrocity. The first and potentially most powerful emotion in combat is fear arising from the instinct of self-preservation. But in soldiers, fear is overcome by what the Greeks called thumos—spiritedness and righteous anger. In the Iliad, thumos, awakened in Achilles by the death of his comrade Patroclus, leads him to quit sulking in his tent and wade into the Trojans.

But unchecked, thumos can engender rage and frenzy. It is the role of leadership, which provides strategic context for killing and enforces discipline, to prevent this outcome. Such leadership was not in evidence at My Lai. We’ll have to see if this was the problem at Haditha.

Under the stress of war, unchecked thumos can push a decent man over the threshold. That’s a fact. But to use Haditha to discredit the efforts of hundreds of thousands of American and Coalition servicemen in Iraq, is as wrong as it was to use My Lai to discredit our sacrifices in Vietnam.

Mackubin T. Owens is an associate dean of academics and a professor of national-security affairs at the Naval War College in Newport, R.I. He is writing a history of U.S. civil-military relations.