The calls for an Easter ceasefire in Kosovo serve to underline the fact that most Christians hold that war is inconsistent with the teachings of the New Testament and that the participation of Christians in any violent conflict must be limited and constrained by the demands for justice and peace.

A leading English Sunday broadsheet has proclaimed: "This is not a just war". But when is a war just? Is the "just war" theory simply a formula to allow Christians to participate in any and all wars? And, in the light of recent events, can the present war against Milosevic and the Yugoslav government be regarded as a "just war"? Over the centuries, the Christian tradition has produced three contrasting approaches to the moral dilemma posed by war: pacifism, Crusades, and the "just war" theory.

Christian pacifism, with its roots in the Sermon on the Mount and the practice of the early church, demands total opposition to all wars. However, the call to peacemaking in the Sermon on the Mount (Matthew 5:9) is not a call to maintaining an unjust cessation of violence or to seeking peace at any price. Nor do the demands of peace always take priority over the demands of justice.

Those who invoke the principles of pacifism in the present conflict are open to condemnation if we have not spoken out against the tyranny of Milosevic, condemned the treatment of refugees and asylum-seekers in this country, or challenged the pervasive antipathy towards Muslims that has allowed the West to ignore the plight of Muslims in Bosnia and ethnic Albanians in Kosovo.

The Crusades found their initial justification as defensive action aimed at protecting Christian pilgrims to Jerusalem, but quickly took on all the characteristics of the jihad or Muslim holy war in which, ironically, Muslims became the victims.

The theological justification of Crusades has long been abandoned, but still has resonances in the way conservative Christians in the US talked about communist eastern Europe during the Cold War, or even today in the way many talk about a looming "clash of civilisations" between the West and the Islamic world.

The third Christian approach has been formulated as the "just war" theory. The concept of a just war owes its original formulation not to Biblical principles but to Aristotle, who first used the term, Cicero and others.

The theory was first framed by St Augustine, who was exercised by the problem of when a Christian might take part in a war with a good conscience.

For him, all wars remained sinful, and war could only be waged in "a mournful spirit". War involved resorting to a lesser evil only in the hope of preventing a greater evil and of restoring justice. He accepted that the command to love our neighbour included a duty to defend the vulnerable against attack, while the commandment to love our enemy placed moral limits on the use of force in defending the vulnerable.

Augustine offered specific criteria for assessing the morality of warfare: the cause must be just, as must be the intention of those who embark on war; war must be declared by the proper, legitimate authority; and the conduct of the war must be just in every particular.

Augustine's theory was later refined and developed in the 13th century by Thomas Aquinas, and by 16th-century Spanish and Dutch theologians. Their formula for a "just war" passed into international law, so that Francisco de Vitoria and Huig de Grotius are known to this day as the "Fathers of International Law".

THE "just war" doctrine does not seek to legitimise, and still less to glorify, war. Today it is widely accepted by jurists and theologians alike that six conditions must govern a decision to go to war (jus ad bellum) and two conditions must govern its conduct (jus in bello). The six conditions governing any decision to go to war are: declaration by a legitimate authority; just cause; right intention; last resort; reasonable hope of success; and a due proportion between the benefits sought and the damage war will bring.

The two conditions which govern the conduct of war are: the principle of proportionality and the guaranteed immunity of non-combatants. Each of the conditions must be met for any conflict to meet the criteria for a "just war".

Is it possible to regard the present conflict as a just war? Of course, it might even be asked whether this is a war. There has been no formal declaration of war; instead, there has been a formal declaration of the beginning of hostilities, and this has not come from a head of state. Whether the Secretary-General of NATO is an appropriate authority is open to question.

Undoubtedly, there is a just cause if the intention is to stop ethnic cleansing. But some British officials have said NATO's aims and intentions are merely to show President Milosevic their resolve. Showing one's determination or flexing one's muscles does not justify bombing city-centres or putting maternity hospitals within firing range: has NATO clearly set out its intentions so that its cause is just?

The refusal of the Belgrade government to sign the Rambouillet accord despite weeks of negotiations may allow many to argue that this war is a last resort. But was there ever a reasonable hope of success? The just-war theory also demands a due proportion between the benefits sought and the damage caused by war. If intensified ethnic cleansing was a foreseen consequence, then surely the sufferings brought about in the past two weeks are out of all proportion to any benefit in bringing Milosevic to the negotiating table.

On the other hand, NATO cannot be blamed for the actions of paramilitary racists, no more than the allied bombers can be blamed for Hitler intensifying the annihilation of Jews in the concentration camps.

Unfortunately, one of the weaknesses of the "just war" theory lies in the fact that 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. 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.

We are left to confess that war is evil, and accept that in circumstances such as these many people of good will resort to a lesser evil in the hope of preventing the perpetration of a greater evil.

Some years ago, when the Harvard theologian, Prof Harvey Cox, was asked who the new enemy of the West would be to replace eastern Europe in the post-Cold War world, he replied: "The poor". Some weeks later the same question was put to the missionary theologian, Bishop Leslie Newbigin, who replied: "Islam". It is ironic that in the present conflict those who are suffering most are both poor and Muslim.

Patrick Comerford is an Irish Times staff journalist and an Anglican lay theologian