Should Nato now revert to its original model: an alliance against a hostile Russia? That is what several countries in Eastern and Central Europe now want.

They won't - and shouldn't - get a response in the very tangible and archaic form that some want. Nato is not about to revert to its incarnation as an organisation that strings forces along the Russian border. It can't afford to, and that would be extravagantly provocative to Russia, without much real value in return.

Nor can they expect it to pull back from Afghanistan, with the security there deteriorating so markedly, and with President Karzai losing political altitude so fast. The “return to basics” brigade might lobby for Nato to avoid new commitments beyond its members' borders but they can't ask for it to jettison that one. If they really want Nato to represent the line between those sympathetic to US and European principles and those who are not, they could ask that the alliance open itself to new members that meet its standards. Unfortunately, this is one decision that the alliance is likely to duck.

Argument about the purpose of Nato is nothing new since the disintegration of the Soviet Union. There have been many pronouncements of its irrelevance. In June 2004 in Istanbul - a Nato summit that in any case was heading for inconclusive debate - Tony Blair and President Bush upstaged the gathering by announcing the handover of Iraqi sovereignty. A coalition of the willing - even just of the US and Britain - was more useful than Nato, they seemed to suggest.

Russia's new hostility might then seem like Nato's salvation: a common cause, for an easily defined group of allies. It should be, but it isn't, because of the ambivalence of some of those right on the border - such as Ukraine - and the desire of Germany, for one, not to antagonise its trading partner and neighbour.

The European Union intention at today's summit with Ukraine is to offer hopeful noises about the chances of joining Nato - and the EU - but to make no promises. That is probably a rehearsal for the same message when Nato foreign ministers meet in December. Britain, one of the most enthusiastic about extending Nato membership, appears to have gone cool on pushing for firm dates.

That is a dangerous game in tantalising Ukraine and Georgia. Europe and the US are fond of holding out the chance of membership of the EU and Nato but don't really want to award those prizes any more, partly because of past casualness in making sure that members were really ready. They would do better to let candidates start the formal application - but set standards that really mean something.

Nato could use Russia's new surliness to redefine itself more clearly. But it needs then to be willing to take in those who share its aims. Unfortunately, the alliance echoes the ambivalence of the EU towards Russia. Most likely, the product of months of analysis of the Russian menace will be an exercise in trying to have it both ways.

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