Philosophy and theories of justice are of no use in the debate about an invasion of Iraq. History and realpolitik are enough and, says John Gray, point to only one course of action.

It is usually a mistake to turn to philosophers for enlightenment about questions of international relations, and that is particularly true now. For much of the past generation, the predominant tendency in philosophy has been to reduce practically all questions about politics to issues of justice or rights and to assume that the rights that human beings are owed are the same everywhere. When liberal universalism of this kind is criticised, it is mainly by thinkers - communitarians and others - who deny the very existence of universal human values.

Neither of these philosophies is much use in helping us think about the coming war in Iraq. The first of them - well exemplified in the work of John Rawls and his disciples - is a version of liberal legalism that, despite much earnest labour, does not square with the realities of the international system. Liberal legalism takes for granted the modern state and the rule of law. It is ill suited to a global environment in which many states have failed and in which the most powerful states are at liberty to act in defiance of international agreements.

On the other hand, unrestricted moral relativism of the sort defended by some communitarian thinkers is not a serious alternative to liberal universalism. Humans have a common nature and many shared needs. This fact could be denied when cultures were more sharply delineated and sealed off from one another by distance and language. Today, when nearly all societies interact with many others and many people belong to more than one way of life, it makes little sense to think of human values as artefacts of discrete cultures.

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The questions we have to consider in thinking about Iraq are simpler, and at the same time in some ways more difficult, than is suggested by these hackneyed debates. It cannot be disputed that there are some universal human values. It is bad for all humans to be at risk of violent death and to be sick or malnourished; it is bad for them to live in anarchy or under tyranny. These are universal human evils; but they do not make possible any kind of universal morality.

In the real world of politics and history that rumbles on beyond the seminars of philosophers, toppling tyranny sometimes means anarchy. At the same time, tyrannical regimes - such as Castro's Cuba, at least before it was impoverished by the Soviet collapse and the continuing American blockade - may ensure high levels of health for their citizens. Universal values are quite often rivals. When they conflict there is no utilitarian calculus, or "theory of justice", that can tell us what to do. Even so, we can think well or badly about how to cope with the dilemmas we face.

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A pre-emptive attack on Iraq carries very high risks. Aside from the extreme suffering that will accompany even a short, limited and highly successful war, there are grave dangers to the people of Iraq, the forces - presumably UK and US - that carry out the attack, and to geopolitical stability in the region. We can gauge the nature and severity of some of these risks by looking to history. When George Bush senior decided to end the Gulf war without marching on Baghdad, he was swayed by three dangers. He feared that Saddam might use the chemical weapons he possessed even then on allied troops; that Saddam might attack Israel; and that in the aftermath of toppling Saddam the state of Iraq might well break up.

All these risks are present today, in equal or greater degree. If Saddam has amassed a large armoury of weapons of mass destruction in the intervening decade or more, he will surely be tempted to use them - especially if he believes he cannot survive the conflict. If he attacks Israel, it will surely - and not unreasonably - retaliate. If Saddam is overthrown, the Iraqi state - a post-imperial construct cobbled together by the British - will face a number of fissiparous forces, not least the Kurds, that may well cause it to fragment.

These are considerable risks, but they do not exhaust the dangerous repercussions of a pre-emptive attack on Iraq. The power of fundamentalist movements in Islamic countries could be strengthened. Governments in countries such as Saudi Arabia and Pakistan - the first critical to world energy supplies, the second a nuclear power - could be threatened, even overthrown, as power shifts to the streets. In a worst-case scenario that may not be too unlikely, a messy and lengthy invasion of Iraq could spark a conflagration in the Gulf and beyond, dealing a devastating blow to the war on terrorism.

With risks as serious as this, the objectives of war on Iraq need to be clearly defined. In fact, there is a good deal of ambiguity about them. Tony Blair has declared that the goal is to enforce disarmament; George W. Bush has made clear that his objectives include regime change. Of course, it can be argued that the two are linked. It may be that with Saddam in power there will never be reasonable certainty that the development of weapons of mass destruction has ceased. On the other hand, history shows that a policy of deterrence and containment has worked. Over more than a decade, the threat of a disproportionate response from the US has prevented Saddam from using such weapons on his neighbours.

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The prospect of Saddam acquiring a nuclear capability is horrific. But is there any good reason to think that deterrence will not work with Saddam as it did with Stalin and Mao - tyrants who were responsible for incomparably more suffering and death and who were not kept from gaining nuclear weapons? If not, what is the justification for a pre-emptive attack on Saddam?

Questions such as these cannot be answered by an appeal to first principles. They require a historically informed assessment of risks and consequences. This may seem a rather obvious point, but it puts paid to the view - currently something close to the received wisdom among liberal political philosophers - that issues to do with war and peace should be decided by courts of international law. This is a non-starter - and not only because the powerful states on whose compliance international institutions finally depend will always disregard them when they believe that vital national interests are endangered.

The liberal legalist view of international affairs implies that the existence of a grave violation of human rights can in itself justify military intervention; but this is nonsensical. It passes over two crucial considerations in any decision to go to war - whether the war stands a reasonable chance of success, and what collateral damage it may incur. These are prudential judgements, which can never be made precise or indisputable; but on them hangs the answer to the dilemma we face over Iraq.

To think clearly about the coming war, we do not need to ponder questions about moral relativism or the foundations of human rights. Nor need we revisit ancient debates about utilitarianism. Engrossing as these subjects may be to philosophers, they are irrelevant to the dilemma at hand. We know that war in Iraq will inflict some great evils. We have reason to fear it may cause even greater evils. We are told that it is necessary to stop the further build-up of weapons of mass destruction; but it is not certain that war is the only way of achieving this, and likely that deterrence will prevent their being used even if the attempt to prevent them being acquired fails.

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Contrary to recent liberal philosophy, the decision to go to war cannot be based on theories of justice or rights. Some such dilemmas may be so ridden with uncertainties that there is no one right solution. But in the case of war over Iraq, I think it is clear that reason is on the side of peace.

John Gray is professor of European thought at the London School of Economics. His latest book is Straw Dogs: Thoughts on Humans and Other Animals , published by Granta, ?12.99.

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