The moral-equivalence crowd is at it again. Many of us recall such arguments during the Cold War: The Soviet Union and the United States are identical dangers, opposite sides of a single coin. Each is expansionist, repressive, and maniacally stockpiling arms. Each wishes to “impose” its way of life on the rest of the world.

When I talked to dissidents in Warsaw and Prague in the 1980s – before the collapse of the Soviet Union – they found the moral equivalence talk mounted by some Western “peace protesters” unintelligible even as they believed Ronald Reagan’s characterization of the Soviet empire as “evil” was entirely apt. The firestorm of protest in some circles in America against Reagan’s rhetoric was utterly perplexing to them. “Why is this so controversial?” they would query, going on to recite the horrors they themselves had confronted under Soviet domination.

Fast-forward to the present moment. Yet again we find equivalences being drawn between terrorists who plant bombs in mass transportation with the intentional aim of slaughtering as many innocents as possible and, by contrast, the tactics of the American military in Iraq. Subway bombers and American soldiers both kill people, so they must be identical. This sort of political stupidity and ethical blindness is mind-boggling.

Since when is it “no different” to take meticulous pains to avoid civilian casualties and, by contrast, to intend and plan for such casualties? That innocents – noncombatants – will come into harm’s way in any war is all too true. But we judge an army by the efforts it makes to minimize such casualties. This is a central feature of the Western just war tradition, which has entered into the operational philosophy of the American military. The two key features of the so-called in bello portion of just war teaching are discrimination and proportionality. Discrimination is the principle at stake here: One must discriminate between combatants and noncombatants. Anyone who claims that the American military indiscriminately kills civilians – and this is central to the moral-equivalence argument – is indulging in cheap polemics, not facts.

Calling terrorists “bombers,” for example, makes this distinction erosion easier. A military “bombs” and terrorists “bomb” so, voila, they are equally bombers. In this way the need to distinguish, discern, and evaluate is removed and mindless moral equivalence becomes easier. “Terrorism,” too, is distorted if it is applied anywhere to anyone fighting for a cause. Such persons may or may not be terrorists. One must first respond to certain questions.

Do armed men (or women) knowingly and purposely murder noncombatants? Do they plant explosives or blow themselves up in cafes, or subways, or buses? Does their rhetoric make no distinction between legitimate and illegitimate targets? One example of such rhetoric would be Osama bin Laden’s insistence that “all Americans everywhere,” men, women, and children, are fair game. Their status as noncombatants matters not at all.

According to the logic of terrorism, “enemies” can be killed no matter what they are doing, where they are, whether they are young or old, male or female, healthy or infirm. It suffices for them to be Jews, or infidels, or “objective enemies.” As Michael Walzer, a leading exponent of just war put it recently: “Terrorists are killers on a rampage.” Soldiers fighting under a rule-governed logic of engagement that incorporates strict criteria concerning legitimate and illegitimate targets are not. For such soldiers, unarmed noncombatants must never be an intended target.

The suicide car bomber in Iraq, a terrorist who steered his vehicle knowingly in the direction of children who had clustered around a U.S. Army patrol and blew himself, and them, up, lives in a world that bears no relationship to that of the soldier fighting under distinctions and rules that would never permit such an atrocity. Innocents will die in any conflict, true. But we attach moral culpability to intended acts or to massive carelessness or recklessness that leads to the deaths of innocents even though they were not intended.

This is something the moral-equivalence demagogues will never understand.

Ms. Elshtain is the Laura Spelman Rockefeller professor of social and political ethics at the University of Chicago and is author most recently of “Just War Against Terror: The Burden of American Power in a Violent World” (Basic Books, 2003).