It was Harold Macmillan, ever shrewd in the ways of politics, who said that the only way to hold a successful summit was to have the communiqué already written before you arrived. On that reading, the Nato summit in Bucharest has all the elements of a truly miserable failure.

The participants are at odds over expansion to the East, with the US, backed by the new entrants, urging Georgian and Ukrainian membership against the public doubts of Germany and the vehement opposition of Russia. The core members are at odds over their individual contributions to the war in Afghan-istan. Even on what should be the relatively uncontentious issue of bringing Macedonia into the organisation, the Greeks are threatening to veto the move unless the new member changes its name.

If this were a family it would compete with the Royal Tenenbaums for disfunctionality. Of course it won't be allowed to end in a climax of slammed doors. The North Atlantic Treaty Organisation is regarded as far too important, and prestigious, for that. Indeed no Nato meeting is complete without a chorus of pronouncements by premiers, politicians and pundits stressing just how important the alliance is to the West and how, despite the end of the Cold War that was its raison d'etre, it is still needed more than ever in the post 9/11 world.

All true, no doubt – or at least in part. Nato has been an extra-ordinarily effective organisation in locking the US into Europe militarily and in containing the Soviet Union. But past pre-eminence is no guide to future purpose and it is the lack of definition of what Nato is for that is now producing all the strains.

With the Cold War the organisation had a defined enemy and a clear function – to defend Western Europe against conventional or nuclear assault. Without the Cold War it has no clear enemy or function, only the persistence of a well-honed military structure. The "War on Terror" proponents – President Bush with Gordon Brown padding along behind him – see that honed structure as a ready-made means of combating the new enemies in a world of Muslim extremism and nuclear proliferation. If Europe was its theatre of operations in the Cold War, Nato's role after 9/11 is, according to this doctrine, to go "out of theatre" to engage in operations in Afghanistan, the Middle East, Africa or wherever else a threat is perceived.

At the same time President Bush, in pursuit of his vision of "democracy" around the world and in search of a legacy for his failing presidency, wants to use Nato membership to secure the new democracies of the Orange and Rose revolutions. Hence his enthusiasm to start the process of entry for Ukraine and Georgia. Add to that a new President of France who wants re-entry to full military participation in Nato as a means to take France back to the heart of international decision-making, and you have more energy for movement in Nato than in a generation.

Only it is an energy without consensus or agreed direction. The reluctance of member states to send more troops to Afghanistan or to send them to the hot spots is not, as Washington would brief, a matter of cowardice or parsimony. It is because, for a number of European countries, there is no public support after Iraq for an operation which makes Nato troops into a white, Western occupying force charged with rooting out drugs, confronting local warlords and instituting civil reconstruction – tasks for which Nato troops are not trained and which make them participants in local rivalries.

In the same way President Bush, and the Ukrainian and Georgian leaders, would make eastward expansion into a matter of facing down Russia. But fear of Russia is not the main reason for German (and French, Belgian and Dutch) doubts. The problem is that expansion this far east would take Nato right into the middle of the conflict between Russian and Ukrainian speaking halves of Ukraine, never mind the problem of the breakaway parts of Georgia, disputes that could easily escalate into confrontation with Moscow. For the very reason that the two countries want membership, the organisation should be wary of it. For, as Moscow not unreasonably argues, if Russia is no longer regarded as the enemy, why are we doing it and in such haste?

This isn't a case of: if we didn't have Nato we'd have to invent it. The opposite is true. If we didn't have Nato we'd invent something quite different at this point. We would be involved in a different way, if at all, in Afghanistan. We'd be using membership of the EU as the means of securing the democracies of the former Soviet republics. And we'd be developing an independent European defence capability.

The fearful prospect at Bucharest is that, by allowing Nato to be driven in new directions without confronting the hard questions on its future, we are in danger of breaking the whole alliance on which it is founded.