Nato will be 60 next year, an age at which many individuals consider voluntary retirement. The alliance, though, is desperate to carry on working. It wants to reinvent itself, in order to face the future confidently, but it is uncertain how. This is the challenge facing those gathering for the Nato summit that starts in Bucharest on Wednesday.

Twenty years ago the idea that such a meeting might be held in Romania - an independent-minded communist state, but a Warsaw Pact member nevertheless - would have seemed ludicrous. Since then Europe has changed so much that the host is not only part of Nato but also of the European Union. This is a tribute to what Nato ultimately achieved in the Cold War. The question is whether it is possible to adapt a body invented for that conflict to play a part in a completely new international order.

At one stage it seemed that Afghanistan would be the disagreement that dominated this meeting. This is Nato's principal military and political responsibility today, but one in which many of its members have been reluctant to be more than symbolically involved. The sense that the burden was being shared inequitably led Canada to threaten that it would scale back its involvement drastically if others would not put men on the ground and commit them to fighting.

A potentially critical split in Nato's ranks has only been averted (or perhaps more realistically postponed) because Nicolas Sarkozy has expressed a willingness to intensify France's involvement in the quest to stabilise Kabul and rout the Taleban. The French President will, thus, be treated as the hero of the hour at the summit. And deservedly so.

While his intervention is welcome there is one drawback. It means that the fundamental question - “What does Nato now exist to do?” - can again be avoided.

That does not mean that Bucharest will be entirely devoid of drama. For there will be debate, on a different question: “Where do the natural boundaries of Nato lie?” The meeting is expected to approve the membership applications of Croatia, Albania and, subject to Greece's objections as to that country's name, Macedonia. This expansion is comparatively uncontroversial.

The standing of Ukraine and Georgia is another matter entirely. Both wish to be considered for the Membership Action Plan, Nato's institutional equivalent of a training camp for membership. The Bush Administration has championed their ambitions but many of its key allies, notably Germany, fear that if this step is taken it would be hard to deny the two countries eventual entry. This is deemed a profoundly provocative move by Moscow. Angela Merkel is not alone in doubting the wisdom of greeting Dmitri Medvedev, the incoming Russian President, in this fashion. A messy compromise might well emerge in which the door to Nato is opened to Kiev and Tblisi, but not so wide that either can force a foot through in an irreversible manner. Such a compromise would be no bad thing.

This episode shows that unless it thinks more deeply about its role and its relationship with its neighbours, Nato risks becoming an ever-larger club with an ever-vaguer mission. Nato summits can be exercises in public backslapping and private bickering. It will not make for a comfortable 60th birthday in 2009 if that pattern persists.