The morality of war has been debated since men fought with sticks and stones. Combatants have wanted to believe that they were not mere brawlers, but took up arms in the service of right.

The current fighting in Yugoslavia is no exception. The Belgrade government appealed to international legal norms this week in asking the World Court in The Hague, Netherlands, to stop NATO's "merciless and savage bombing of the civilian population." And Western leaders have justified those airstrikes as motivated by moral and humanitarian concerns, not national self-interest.

World Wide Web sites, law school journals, theologians and opinion writers have all weighed in. Though there is much disagreement, a plurality seems to have formed around the idea that the NATO action may have been morally legitimate in its stated aim of stopping "ethnic cleansing" in Kosovo, but it has lost its justification by failing to achieve that aim.

"Not only has NATO's bombardment failed to stop the ethnic and religious 'cleansing,' " read a Web site posting under the name of "Imemind," "but the air war also seems to have intensified the problem."

Thinkers since Aristotle and Cicero have pondered whether war may ever be just or necessary, but modern theories can be traced to the early Christian era.

Following the command of Jesus to be peacemakers, early Christians believed that they should not fight in wars. But, regarding themselves as citizens of an eternal spiritual kingdom, they also generally refused to participate in the civil administration of the Roman Empire.

Then in 313, on the eve of a battle, the Emperor Constantine had a dream. He saw a cross and the words, "In this sign you will conquer." After his victory the next day, he converted to Christianity.

With theirs now Rome's official religion, Christians had to rethink their role in the life of a secular state. The most systematic explication came from Augustine of Hippo (356-430), who wrote that the civil state was divinely ordained for the earthly protection and well-being of its citizens, in parallel to the spiritual kingdom that looked after Christian souls.

It followed, according to Augustine, that war could be undertaken when necessary to protect the peace and good order of a society -- but not for "the desire of harming, the cruelty of revenge, the restless and implacable mind, the savageness of revolting, the lust for dominating and similar things."

As the principles have developed, wars must meet seven tests. They must be waged by a legitimate authority, for a just cause and with the intention of securing a just peace. Fighting must be a last resort. The means used to wage the war must be proportionate to the goal and must discriminate among legitimate targets. Finally, there must be a reasonable prospect of achieving the just aim.

Simple as these principles seem, they have given rise to endless interpretation.

Could Christian priests fight, even in a just war? Most commentators thought not, but at the Battle of Hastings in 1066, Bishop Odo of Bayeux took up a mace, not a sword, so that he could kill without shedding blood.

Can Christians fight on Sundays or holy days? Is it unpardonably deceptive to lay ambushes? Thomas Aquinas (1227-1274) sanctioned both practices.

Is it possible to discriminate between fighters and noncombatants in a guerrilla war, where warriors may live among a civilian population and wear civilian clothing? And, in fact, are not civilians often the architects of war and soldiers merely the instruments?

NATO's attack on Yugoslavia has offered new questions for interpretation. Most discussion accepts that prevention of ethnic cleansing in Kosovo is a "just cause" for war.

But some theologians, such as Stanley Hauerwas, a professor at Duke University, scoff that the idea of "just war" makes no more sense than "just adultery."

The Rev. Al Humbrecht of Sacred Heart Cathedral in Washington suggests that if war was ever justifiable, it no longer is.

"With the type of weapons we have today, I think not," he told an interviewer. "In an age when war was hand-to-hand combat, St. Augustine could talk differently than in an age when we use 'smart bombs.' "

Others question whether pacifism is an adequate moral stance in the face of evil.

"This war, more than most, confronts Christians with choosing between saying 'never again' to war or saying 'never again' to ethnic cleansing and genocide," wrote the Rev. Thomas J. Reese in the magazine America. "These are choices we don't like to make."

As the war has unfolded, some writers who have supported other wars question whether this one is really about Kosovo. Conservative columnist Tony Snow quoted the rhetoric of NATO leaders who invoked "the consensus of the international community" and "the need to ensure the viability of NATO."

"We fight not to protect or defend," Snow wrote, "but to improve as we see fit. ... Our commander in chief has embraced the explicit goal of killing people so their leaders will behave and NATO may enjoy a modicum of self-esteem."

The question of ulterior purpose, or "just intention," has long vexed just-war theoreticians. In calling a few years ago for Western military intervention in Bosnia, the National Council of Churches tried to distinguish the case from the 1991 Persian Gulf war against Iraq, which it had condemned.

The difference was that the politics of oil was inextricably mixed with the liberation of the conquered Kuwaiti people, whereas the relief of Bosnia could be justified as motivated exclusively by humanitarian concern.

The principal objection to the air campaign in Yugoslavia, however, has been that it seems increasingly unlikely to be effective without using ground forces, which NATO ruled out in advance.

"If intensified ethnic cleansing was a foreseen consequence," wrote Patrick Comerford, identified as an Anglican lay theologian, in the Irish Times, "then surely the sufferings brought about in the past two weeks are out of all proportion to any benefit in bringing [Yugoslavia's President Slobodan] Milosevic to the negotiating table."

In an Internet discussion, Anthony Buckland complained that "NATO is not trying to win the war. ... According to just-war theory, one must have not only just cause, but also effectual means and will. For a war to be just it is not enough that one is fighting evil people; one must also have a plausible prospect of putting an end to the evil acts of these evil people."

The president's own pastor is still behind him, according to an interview with Scripps Howard News Service. The Rev. Philip J. Wogaman of the Foundry Methodist Church in Washington says, "We are at a time in world history when the resolve of the international community may be necessary to prevent serious abuses, like genocide, in particular places, and thereby prevent them from expanding."

Comerford acknowledged "one of the weaknesses of the 'just war' theory":

"It is often only long after a war is over that we have the time and the luxury to determine whether all conditions were met," he wrote. "In the meantime, we can only accept that all our moral decisions are contingent and at best penultimate, rather than having ultimate or final value."

The Just-war theory

Here are the seven classical tests in just-war theory:

Right reason. The war must be undertaken in self-defense or to recover land or goods unjustly taken.

Right authority. It must be waged by a legitimate government, not by groups or citizens taking the law into their own hands.

Right intention. The goal of war must be the restoration of peace, not conquest or power.

Last resort. Every reasonable diplomatic effort must be exhausted.

Likelihood of success. War is not justified if defeat or stalemate is the likely outcome.

Proportionality. The good to be secured by the war must outweigh the harm done by waging it.

Discrimination. Noncombatants should not be attacked nor prisoners mistreated.