The drive towards war on Iraq appears unstoppable, but it's not too late to pause to reflect on the justice of the fight - a king once halted a war so his leading thinkers could consider the very questions we face today, says Richard Sorabji. What's more, argues Bill Durodi¨¦, soldiers need conviction to be fit to fight; although Peter Sluglett says the case is clear: Iraqis need to be freed of a despotic leader

In 1550, the most powerful sovereign of the age called a halt to war. The conquistadors were in the process of conquering South America when Charles V, Roman emperor and king of Spain, suspended the conflict to give the kingdom's thinkers time to conduct a philosophical inquiry into the justice of the fighting. The debate was held before a jury of theologians, philosophers and counsellors. Although it ultimately did not stop the conquest, which had started soon after Columbus' discovery of America in 1492, it should impress and inspire us that the question of justice should have been taken so seriously as to create a year's halt, and should have given such a role to philosophical debate.

The American Indians were championed by Bartolom¨¦ de Las Casas, an impassioned Dominican priest who had spent time in South America and had benefited from the conquests before converting to the Indians' cause. He was a contemporary of another leading thinker and defender of the Indians, fellow Dominican Vitoria, a university professor in Paris and later Spain, whose work had an enormous influence on thought about law and human rights.

These two important philosophers tackled the very questions that we are having to address now, and their ideas offer us useful tools to think with about Iraq and many other conflicts.

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This is not to agree with all their decisions, which are sometimes opposed, nor to say that their situation was the same as ours. In one way, it was much easier for them as they had not been the victims of a horrifying massacre of innocent civilians, contrary to all the rules of justice that will be mentioned below. But we must remember that Iraq took no part in the 9/11 massacre. What is important is that the Spaniards were asking questions that we must ask, too.

Had one the right to rescue the victims of a foreign regime? Some American Indians practised human sacrifice. Vitoria believed that you could save the victims by invading, but Las Casas offered two important reasons against.

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First, you must consider whether you would kill more non-combatants than were going to be sacrificed. Second, you must consider an invasion's adverse psychological effect on your ultimate aims. For Las Casas, this meant the willing conversion of the American Indians to Christianity.

What about pre-emptive killing in a just war? Vitoria ruled out the pre-emptive killing of non-combatants who may be dangerous later on the ground that we do not allow pre-emptive execution in civil society. But he conceded that combatants who remained opposed to you at the end of a war that had been entered into justly might be executed if they still posed a threat.

Could non-combatants be killed in a just war? Both thinkers thought not.

But women and children would naturally shelter in fortresses, and visiting merchants would be indistinguishable from enemies. Here the Spaniards' rules are strict: the war must have been entered into justly, taking the fortress must be essential to your war aims, and finally, the harm done must not outweigh the good.

Could one require regime change in a just war? Even if a war had been entered into justly, Vitoria thought that regime change would be excessive except when the number and atrocity of the injuries already done to you by this regime were great, especially if these injuries might be resumed, and when the regime was not acting in good faith.

These thinkers were speaking in a tradition of justice in war that stretched back nearly 2,000 years to Plato, Aristotle and Cicero and to the book of Deuteronomy among the Jews. In fact, all parties to the conflicts that most worry us now have very long traditions of considering this matter. This is true also of Islam and of India, whose ancient epic, the Mahabharata , is full of such considerations.

The early Christian thinkers were embarrassed by the apparent contrast between the approval of war in the Old Testament and Christ's injunction to turn the other cheek. Christians such as Tertullian and Origen in the first half of the 3rd century AD dismissed the Old Testament and thought that Christians must be pacifists. But St Augustine after 400AD explained away the New Testament: Christians may fight, but this called for thought about when it would be just to do so. Sometimes, five criteria are picked out of what Christians said:

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* There must be a just cause for war, such as righting an injustice

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* All peaceful methods of resolution must have been tried first

* The right authority must declare war

* You must be sure that war would not make things worse than they already were

* Your war aims must be achievable.

A second major question surrounded just conduct after war had been entered into. This was of concern not only to Christians but also to the Greeks, Romans, Jews and Indians, while Islamic legal texts considered the treatment of women and children during sieges, the use of Muslims as human shields and suicide versus martyrdom. The Hague and Geneva conventions that focus on just conduct within a war are latecomers from the 19th and 20th centuries.

A powerful attempt to advance just conduct within war was made in the 12th century, again by two thinkers from Spain - Averroes from the ruling Muslims in C"rdoba and Maimonides from the subject Jews. Averroes merged his Islamic beliefs with ancient Greek philosophy to form a unitary tradition. He used Aristotle's ideas about judicial discretion to argue that the imam must be left free to modify the duty of Muslims to kill polytheists. Maimonides, who was equally steeped in these traditions, commented on Deuteronomy and other books of the Torah. Speaking of conduct within war, he approved such modifications as besieging a city only on three sides so that non-combatants could escape from the fourth. This tends to happen in modern wars as refugees flee an attacked country in whatever direction they can. But we regard this as a problem, not a solution.

The psychological impact of appearing unjust is known not only in the arena of war considered by Las Casas, but also in the context of combating terrorism. Justice may be more powerful than weapons.

As for pre-emptive measures, they are already an issue for us as much as they were for Vitoria. In addition to the proposed invasion, US and UK laws since 9/11 allow pre-emptive imprisonment of non-combatant suspects. We may also consider the pre-emptive character of the detention of Afghan combatants in Guantanamo Bay and the use of unmanned planes to assassinate suspects in countries not at war, such as Yemen.

The five criteria devised by Christian thinkers can be applied to today's situation with Iraq. We have to ask whether an attempt at peaceful resolution counts if we attend to recalcitrance selectively in some Middle East regions and not others. Most people think the UN is the right authority for declaring war in the case of Iraq. And the achievability of war aims requires careful thought, insofar as we aim to reduce world terrorism and the risk that weapons of mass destruction will be put to use.

The remaining topic, just conduct within a war, has now taken on dimensions that Las Casas and Vitoria could not foresee. But their thinking gives us clear answers - there would have been no question of using tactical nuclear weapons or landmines, since these could harm innocent people long after the war is over.

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Richard Sorabji is Gresham professor of rhetoric at Gresham College, London.