The use of force is barbarous unless it is in support of some concept of order. We are reluctant to go to war for oil, for profit, for territorial gain or for conquest pure. To justify violence, we invoke the stability of the international system, the sacred soil of the motherland, the historic destiny of the nation, the rule of international law, the desirability of making the world safe for democracy, civilisation, socialism or something similar. We talk of sacrifice in war precisely because violence is, or should be, sacred.

Even when, in retrospect, we judge that the use of force has been wrong - either because it was mistaken or because it was illegitimate - it has been justified with reference to some higher goal. The Soviet Union invaded Afghanistan to support the spread of a new socialist world. Britain and France defended their action over Suez to protect the world from tyrants who were ready to take the law into their own hands and seize private property. The US fought in Vietnam to prevent the spread of communism through south-east Asia and the world. At other times, wars have been fought to preserve the balance of power, to enforce the doctrines of the Church, to spread the teachings of the Prophet, to assert legitimate claims on an inheritance, or to create a new order in Europe or Asia.

There may be occasions when force is used without such a justification. Perhaps the violence of the Revolutionary United Front in Sierra Leone comes into this category. But even in these cases, as often as not, some spurious tale is told of magical powers or tribal revenge. Very likely, when the barbarians sacked Rome, they did so in the name of ancient liberties or the honour of the clan.

Today, when we have lost faith in nation, God and history, we prefer to make war in the name of law or of humanity. Or we may do so for the preservation of an international order that is supported by law and which tries to operate for the good of humanity.

Every age remakes the law according to its needs and its values. Domestic laws and constitutions change to meet the needs of the times. In the past hundred years, the kings and emperors have gone, retired from politics to their palaces, deserting the pages of the serious press for the world of fashion magazines. In most advanced countries, women have acquired new rights. Racial discrimination has become illegal; abortion and homosexual practices are increasingly permitted. New crimes such as hacking have been created to match the new technology, just as copyright was invented to take account of the coming of print.

In the same way, notions of international law change to match the changing world. The idea of the state as sovereign (replacing the universal jurisdiction of the Church), the laws of war, the outlawing of piracy, the abolition of the slave trade, the concept of self-determination and the doctrine of human rights have contributed to a growing idea of international law. The last two of these are perhaps especially significant, since they represent the beginning of the encroachment of international law on the domestic sector.

Military intervention is justified if it represents a kind of self-defence by the international system, if its objective is to preserve the values and order that enable men and nations to live and go about their business. It is, therefore, important to ask what sort of international order we live under. The answer is that the world we live in today has two striking characteristics, both with important consequences for the legal order. First, it is a world driven by a global economy; second, it is a post-imperial world.

Globalisation needs peace, order and a framework of law. Just as the nation state grew largely to allow orderly economic exchanges within particular boundaries, so international institutions are growing now to allow trade to flourish across boundaries. International trade needs some minimum system of rules: both the rules of international contracts that the private sector has developed and also the framework of rules created by bodies such as the World Trade Organisation, which provide some security for international companies against unpredictable state interventions. To this should be added international law on patents, navigation, exploitation of natural resources, finance, air travel and much else on which the health and prosperity of the world economy depends. Then there is the gigantic body of "regional law" developed by the European Union, which, although it operates at a level higher than that of the nation state, does not pretend to universality.

The central point of globalisation is that it has eroded the distinction between internal and external. Economies that are open to trade and investment are also open to influence from outside. Industrial changes overseas impact on jobs and profits at home. Crimes committed abroad - drug smuggling, people smuggling, waste dumping, money laundering - may ultimately endanger your safety. In an open world system, the risk of the ill effects of war spilling over are much greater than ever before: investments abroad may be lost; domestic social security systems may have to cope with large flows of refugees. At the same time, the growth of international media brings into our homes the pity of war to an extent never known before.

All of this creates an environment in which the pressure for external intervention is greater than ever before. The characteristic post-cold war conflict is a civil war; and the characteristic intervention is not, as in the past, intervention for conquest but a peacekeeping operation designed to bring a civil war to an end or to solidify a shaky peace. In keeping with the reducing distance between the internal and the external, such missions may include police as well as soldiers, and even judges and prison officers. Indeed, we find ourselves describing a military operation as a "police action" and prosecuting people for war crimes. The purpose of a peacekeeping operation (of the Bosnian variety, for example) is to protect civilians, to restore order and to allow ordinary life to go on. In this sense, it is quite different from what, historically, armies have usually done - to attack other armies and to disrupt ordinary life as much as possible.

And, just as the borderless economy undermines the economic power of the state in favour of the individual, so our society's emphasis on individual rights and freedoms begins to threaten the idea of state sovereignty as the sole basis for the international order. Gross violation of human rights has been added to self-defence and to UN security council resolutions as a legal justification for intervention. As Kofi Annan argued following the Nato campaign in Kosovo, sovereignty cannot be protected if the result is that the sovereign attacks his own people. Taking this argument a stage further, Richard Haass, director of policy planning at the State Department, has suggested that sovereignty implies obligations as well as rights and that the rights may be forfeited if the obligations are neglected. This might be described as a shift from a Grotian world (after Hugo Grotius, the 17th-century Dutchman regarded as the father of international law), where the object of international law was the state, to a Kantian one, where the law exists to protect individuals rather than states. The corollary of a willingness to contemplate intervention abroad where we think a state has exceeded international norms is a readiness to condemn, and occasionally to take action against the use of violence at home - in Zimbabwe or in Chechnya, say.

The rule of law is what allows globalisation to function. And in a world ruled by law it is no surprise to find that lawyers are increasingly consulted about matters that were once the exclusive concern of the military: today's policy-makers have to ask not just whether an operation is militarily feasible but also whether it is legally defensible. This is one important difference from the world of the cold war: questions of law did not belong in a life-and-death struggle.

And yet, there is a nagging doubt here. It is by all means reasonable to ask of certain government actions - the disposal of nuclear waste or the imposition of a steel tariff - whether they are legal. Applied to a military intervention, the question seems beside the point. Legal advice may occasionally result in a changed target, but would it ever persuade a government to change its mind about taking action altogether?

International order is created by force, preserved by force and backed by the threat of force. International law is the expression of the order that allows us to run a global economy; but force is what established that order. Force may be legitimate or illegitimate; it may be wise or foolish; it may be in the interests of the international community or not; but questions about whether it is legal or not seem - at this stage of world history, at least - merely pedantic. In domestic affairs, one does not ask if a constitution is legal or not.

This does not imply that law and force are alien to one another. Our order is above all a legal order. If force can be used within a legal framework, so much the better. But even where it does not make sense to question its legality, it should still be used in support of the law - to support the civil power, say, or to establish an environment in which the rule of law becomes possible.

The second characteristic of our age is that it is post-imperial. A hundred years ago, the surface of the earth was largely occupied by imperial powers and their possessions. All the empires are gone now: the Ottoman empire, the Austro-Hungarian, the German, British, French and Japanese empires, and finally the Soviet empire. Not only have the empires disappeared, but with them has gone the will to conquer and to rule in other lands. One result of this has been the appearance of so-called failed states: countries that would once have been ripe for imperial takeover but which today nobody wants to take on, and which fall into civil war or are taken over by criminal elements. Most of the international interventions in the post-cold war period have been in failed or failing states: Bosnia, Kosovo, Macedonia, Somalia, Afghanistan, East Timor. (The exception was the Gulf war, an intervention of a much more traditional kind, where the threat posed to the world order of state sovereignty was also of a more traditional variety.) For the moment, the international community continues to attribute sovereignty to states even when they have failed. One wonders how long this doctrine is going to last? It is paradoxical to consider as sovereign a place where no government exists to exercise that sovereignty.

These interventions have been characterised by two features of the post-imperial style. First, they have been multilateral. If a single state intervened, it might be suspected of imperial motives; multilateralism provides some limited guarantee that this is not the case. Multilateralism does not give a full guarantee of legitimacy, but it helps.

Multilateral institutions are in some respects the successors to now defunct empires as a way of organising the world. They provide the framework for trade, investment, cultural exchange and the rule of international law that was once furnished (up to a point) by imperial structures. Especially striking is the post-imperial style of intervention employed by the European Union around its borders, and in particular in the Balkans. Coercive military intervention is the most dramatic form of intervention but it is by no means the most frequent: advice, political and economic pressure, monitoring (often in the context of possible eventual membership of the EU), are all employed. All these are methods of influencing domestic decisions and all have played important roles in reshaping post-cold war Europe.

Regional law and regional norms may be more important than supposedly universal laws. Western intervention in Kosovo was justified formally in terms of international humanitarian law; in practice, it is doubtful if Nato would have felt able to make a similar intervention in Africa or Asia. It is not just that the Nato countries would have had more post-colonial suspicion to live down, reducing the legitimacy, if not the legality, of action. They would also have lacked the historical references that gave the action in Kosovo much of its legitimacy: the Holocaust and the ethnic cleansing of the Second World War. It was in these terms, rather than in the language of international humanitarian law, that western politicians explained their actions and which their publics understood them. Thus, although the Enlightenment tradition makes us think of international law as universal, in practice, there is probably a strong regional and historical element in how we think of the legitimacy of international intervention; and, perhaps, in due course, this should be incorporated into our thinking about law.

The era we live in today - the post-cold war world of globalisation - is still young. The system of law we have begun to create is designed to make the world safe for commerce and (the two are not unrelated) to promote human rights. The world we are trying to shape seems a rather peaceful, rational construct in which more will die from obesity than from violence. (This is already the case in the United States.) We may never get there.

The events of 11 September have shown us the other side of globalisation and the post-imperial world. We live in a world that is doubly vulnerable: first, because it is open and cross-border trade, travel and communication has never been easier; second, because an international division of labour, in an ever more competitive global economy, forces us to operate on increasingly fine margins of error. It requires much less to do serious economic damage to today's world than was the case 30 years ago. On 11 September, it became clear how much damage a small group can do to our society. At the same time, it provided a powerful image that will dominate the imagination of the disaffected for decades to come. In the second half of the 20th century, the east Asian imagination was possessed by the idea of imitating Japan, and building a state and economy to rival that of America. In this century, the consuming passion of the discontented and the dispossessed in the Middle East, and possibly beyond, may be to reproduce the destruction of 11 September.

Globalisation represents a radical redistribution of power away from the state (which remains confined by borders and by national culture) and in favour of the private, the corporation, the non-governmental organisation, the individual, the criminal - anyone who can organise themselves to operate in a borderless environment. Where the external and the internal merge, large-scale crime may begin to resemble small-scale war - the two coming together neatly in the concept of terrorism. And a (Kantian) world that empowers individuals also empowers terrorists.

Simultaneously, a second redistribution of power is taking place through the development of relatively cheap and powerful weapons. Since artillery first became important, it has been quite easy for governments to maintain a monopoly on the means of violence. Private organisations and individuals could not raise the revenue required to organise a train of siege artillery. Nor (short of recruiting an army, which has never been a real possibility outside the chaotic context of the Thirty Years War) could they muster the destructive power needed to do serious harm to a state or society - particularly agricultural societies, which, being largely self-sufficient on a village level, are highly resilient, as the US discovered in Vietnam. Today, however, the possibilities of attack through chemical, biological or electronic means are increasingly available to individuals or small groups. In an open society, neither the knowledge nor the material required can be kept only in the hands of governments. The events of 11 September have shown what can be done without using any of the purpose-built technologies of mass destruction; future attacks may be even more devastating. The Bush administration's twin focus on terrorism and on weapons of mass destruction is precisely right. We are lucky that the instances of the two coming together are, so far, relatively limited. We cannot count on this continuing.

If these threats develop - and history suggests there is good reason to be pessimistic - then both internal and international order will be at risk. How the international system will cope remains to be seen, but it is unlikely that the comparatively benign, ordered, law-governed world that seemed to be emerging from the cold war will survive intact. If the state and the state system weaken, if violence becomes cheap and the states lose their monopoly on it, if disorder grows, the prospects for international society look increasingly bleak. This does not mean that we must give up the attempt to govern force by law. A law-governed society of nations still remains our long-term objective. None of the alternatives (government by men, God, history, race or power) seems attractive. It means only that our chances of success will be limited unless we find the means to control the forces set free by technology. In the meantime, we have to adjust our legal concepts to take account of the new realities: pre-emptive force and covert operations may need legal recognition; and the UN security council may or may not be able to play a central role. At all events, we cannot continue as though nothing is happening, and assume that present conceptions of the legal order must remain unchanged.

Robert Cooper is director-general for external and politico-military affairs at the Council of the European Union, and a former adviser to Tony Blair. The views expressed here, however, are entirely personal. His essay originally appeared on the Crimes of War website, www.crimesofwar.org