The issue is American power. It has been the issue at the back of every big crisis since the end of the cold war. Is the United States bound to assert its dominant position, insisting that its interests must take precedence over all others? Or can this power be harnessed for the international good, not only respecting the rights of other countries but also listening hard to what they have to say?
The issue takes one form when the Americans become hyperactive; it takes another when they are lethargic. Although the current anti-war imagery portrays a US on the rampage, fired up by dreams of world domination and guided by doctrines of pre- emption, the overall impression was quite different for much of the 1990s, especially following the debacle in Somalia. Recall the Clinton administration's desperate attempts to describe the slaughter in Rwanda in 1994 as anything but "genocide" so that it would be absolved from any obligation to stop it.
Tony Blair and Jacques Chirac hold divergent views about how to manage American power. Yet their starting points are remarkably similar. Blair, like all British prime ministers, wants to keep the relationship with the US special, but he has never had illusions about the unilateralist tendencies of both Bill Clinton and George W Bush. His strategy has been to stay as close as possible to the US in order to demonstrate the value of having allies, and the possibilities of multilateralism.
Blair hoped that 9/11 could be used to prove the need for an active internationalism, addressing conflicts in the developing world before they reach boiling point. By taking the question of disarming Iraq through the Security Council, he sought to strengthen the United Nations and demonstrate to Bush its value. Last year this strategy appeared to be working a treat. Now, it has been undermined not only by American readiness to let a military timetable dictate a diplomatic timetable, but also by Chirac's quite contrary strategy.
The French aspiration, which dates back to Gaullist times, is to balance American power in what Chirac calls a "multipolar world". In the past, France, accepting that it could not do this alone, described it as a mission for Europe. The rest of Europe, however, was never really interested, being largely comfortable with the American alliance, which minimised the need for European defence spending and which seemed to work in containing the Soviet Union. After the end of the cold war, with the power gap growing all the time, the aspiration seemed even less credible.
With the normally pro-American Germans taking a hard anti-war stance, the Iraq crisis gave Chirac an opportunity to revive the old Franco-German axis as the motor of Europe. But with Germany weak and Europe more diverse, that notion has had its day. And while Russia has its own reasons to be wary of US hegemony, its basic priorities are at home, and any solutions need American support. Reviving the old Gaullist dream of developing an international constituency based on radical Arab regimes and francophone African states is also a non-runner.
France's obvious collaborator is Britain, the country to which it is closest in outlook, capabilities and interests. For much of the 1990s, it looked as if the two together could develop an effective, European-based (if not quite a European) foreign policy. At St Malo in 1998, Blair and Chirac launched a joint initiative to develop a more serious European defence capability. From the start there was tension between Blair's view that the need for a greater European capability was to demonstrate to the Americans that it was worth them working with their European allies, and the French view that it was the opportunity to create an alternative to Nato. The compromise held because the EU had insufficient resources to duplicate Nato's planning and command capabilities, while Blair accepted that in certain contingencies involving neighbourhood crises, the Europeans might need to operate without the Americans. The problem has been that the extra military capabilities required to demonstrate that this is a real change in European policy, rather than yet more institutional fiddling, are yet to materialise and are unlikely to do so, especially given Germany's budgetary difficulties.
Actual engagement in military operations is a minority taste in Europe, especially as involvement in most modern wars appears discretionary. Conflicts close to home, as in the Balkans, demand some response, but even that was half-hearted when Europe was acting without much US support in the early 1990s. The Atlantic alliance was based on an unambiguous, existential threat. The world now seems much more complex and the choices correspondingly more difficult. As a result of enlargement, both the EU and Nato are becoming unwieldy. The problem is not whether the EU can replace Nato, as the debate was characterised two years ago, but whether either institution has a major external role at all.
With the notable exception of Kosovo, which was fought by Nato, and won despite the alliance's strategy, the key military operations of the past decade have been fought by coalitions, with or without the blessing of the UN. When the US secretary of defence, Donald Rumsfeld, observed in the Afghan context that the mission should determine the alliance, rather than the alliance the mission, he caused dismay in Europe. But in many respects it was a reflection of the truth, as was his suggestion that the US could fight the Iraqis without the British if necessary. He did not want the French involved in Afghanistan because he assumed they would demand a major political input in return for a minor military output (which is how the Pentagon viewed the Kosovo experience). As the range of US weaponry gets longer and longer, there is less interest in basing forces abroad. The US has given both Germany and South Korea notice that if they really do not like American forces in their countries then they will happily leave, and can do so without necessarily jeopardising US security.
So Bush may no longer be interested in coalitions of the willing - even if they are able - while the attempt to buy allies, as in the case of Turkey (a coalition of the billing) has gone down badly in the US. That at best leaves coalitions of the compliant.
It is hard to imagine Bush being so readily persuaded to take difficult issues to the UN Security Council if the result is either paralysis or the US in the dock. This might not matter if the UN, or the EU, was strong enough to manage issues without the US, but it is not. No other power can enforce UN resolutions.
There is a belief in some parts of Europe that the methods used to transcend Franco-German antagonism - soft power and institution-building - can work for the rest of the world. But like all the more optimistic approaches to foreign policy, these methods work only until they get to the hard cases - the countries that persecute their own people, develop and prepare to use the most destructive weapons, and provide havens for terrorists. With the US, the record is patchy; without the US, it is deplorable.
If the war goes as badly as Chirac, Gerhard Schroder and Vladimir Putin have warned, they may feel vindicated but they will also have to tolerate Britain and the US entering a period of caution if not complete withdrawal. It is not as if these leaders have an agreed programme for dealing with the many international problems that are left, and a resentful US, in a stabbed-in-the-back mood, could withdraw co-operation on a whole range of international issues. Those who believe that the US is already hopelessly unilateralist have no idea how bloody-minded a truly isolationist America could be. If Britain gets burnt in a military adventure with the US it is unlikely to opt for different types of adventures with Europe. British politics, probably with a new prime minister, will enter an inward-looking phase.
If the war with Iraq has an early and positive conclusion, Blair may be able to stabilise his position and claim another stride towards a new and better international order. He will have helped rid the world of an obnoxious regime. But his secondary objective - of getting the US into the habit of realising its foreign policy objectives by working through international organisations - will still have suffered a severe setback. It would be nice to think that the past few weeks have been salutary for Bush, that he has learnt that even well-disposed allies do not like being taken for granted, and that effective coalitions have to be built through active diplomacy rather than peremptorily summoned - but that does not seem to be the current American temper.
To rebalance his foreign policy, Blair will need to turn again to France. It is difficult to overstate the bitterness felt in both London and Paris: Blair at the French failure to engage with his strategy, Chirac at the British eagerness to blame him for blocking a resolution that enjoyed only minimal support in the first place. A strengthened Blair may think that it is the right time to join the euro and that this will be a significant political gesture to demonstrate his European credentials. But it will not be a substitute for a more direct approach to Paris; Blair would risk Chirac replaying Charles de Gaulle's "non" to European Common Market entry for Britain in 1963.
Nor should Blair place too much hope in the inevitable schemes and clever designs that the international foreign policy elite will offer, possibly involving new transatlantic institutions with some grand compromise at the core. If recent history tells us anything, it is that the critical relations between the major powers are shaped less by constitutional conferences and agenda-setting summits than by the pressure of events. We will be living with the consequences of the mismanagement of the Iraq crisis of 2003 at least until the next crisis comes along.
None the less, there are two obvious and immediate steps that can be taken to revive Franco-British co-operation. The first concerns the post-Saddam reconstruction of Iraq. Rightly or wrongly, the US and British planners have always assumed that winning the war would be the easier part. Preventing the break-up of the country and reviving its battered economy and depressed political system will be much more of a challenge. It has also been assumed that the UN and the other major powers, given no second resolution, will be inclined to let the British and Americans get on with washing up their own dirty dishes. Yet Russia and France also have interests in Iraq and may be nervous about seeing the country dominated by the US and Britain. And if there is one area where the EU has the capacity and the relevant experience it is in post-conflict reconstruction. A collaborative effort to help Iraq towards recovery and whatever sort of democracy is feasible provides one means of repairing the damage done to the western alliance.
The second issue is the Arab-Israeli dispute. However cynical it may seem for Bush to promote the "road map to peace" on the eve of war, this is one area where the US president has made clear promises. For Blair, it will be a critical test of whether he really does have influence over this administration. A Franco-British plan, requiring action from Europe as well as the US, may provide another means of moving the issue forward, by getting the parties to talk about specific proposals and not just broad principles.
That still leaves the question of American power unresolved and there it is likely to remain for some years. The lesson of the past 12 months is that the United States follows its own agenda. Blair's case is that it is more likely to be redirected through gentle persuasion than through outright opposition. But it is hard to make great claims either way for the degree of redirection possible. Blair and Chirac need to return to the compromise understanding of St Malo. Whether the aim is to prepare to act as a close ally or a critical friend of the United States, far less is going to be achieved when Britain and France act in opposition to each other than when they act together.
Lawrence Freedman is head of the school of social science and public policy at King's College London and the author of Kennedy's Wars (2000) and editor of Superterrorism (2002)