Fifty years after it was founded in 1949 Nato celebrated its anniversary with optimism and panache. A summit in Washington extolled the commitment, relevance and enduring legacy of an organisation dedicated to protecting democracy, pluralism and Western values from the aggressive advance of Soviet totalitarianism.

True, Nato was in the midst of a controversial war, dropping thousands of bombs on Serb forces in the attempt to evict them from Kosovo. The splits on tactics and aims were deep, but in 1999 Nato was still united on its principles: to protect nations from aggression, to confront dictatorship and stop Moscow imposing its will on weaker neighbours. So attractive were these principles that former Warsaw Pact members were clamouring to join: Poland, the Czech Republic and Hungary became full members within three months. Nato, it seemed, had found a new role as the guarantor of regional peace and security throughout Europe.

How different it looks nine years later. Nato has almost doubled in size but its new role as a regional peacekeeper has hardly been strengthened: if anything, the inclusion of new members has confused its purpose. When the Berlin Wall collapsed, Nato insisted that the halting of a Soviet attack was no longer its main military mission; instead, it was to set up rapid reaction forces to deal with threats to European stability. The new entrants, however, saw Nato very differently: it was to be their political as well as military shield, protecting them from any pressure from the East. Nato remained, in their eyes, an anti-Russian alliance - a view that nationalist politicians in Estonia or Poland have expressed to the embarrassment of older members. Little wonder that Moscow remains suspicious and has never embraced what seemed the mutual consultation offered.

Nevertheless, the secondary benefits seemed to remain secure: to co-ordinate European defence and prevent each member adopting divergent policies; and to ensure the continued transatlantic engagement of Canada and the United States.

Both assumptions now look doubtful. Two things have shaken the Atlantic solidarity: 9/11 and Afghanistan. The attacks in September 2001 prompted the invocation, for the first time, of Clause 5, which insists that an attack on one is an attack on all.

But solidarity was not what a shocked Bush Administration sought. It wanted instead carte blanche to protect America without reluctant partners demanding consultation and prior warning. Instead, the Pentagon planned its response untrammelled by its alliance; to some hawks, indeed, Nato seemed to have outlived its usefulness.

The mood fuelled a quarrel going on since the Reagan era on burden-sharing. The Europeans, believing the Cold War over, were demanding a hefty peace dividend. But Washington had already made clear that others should pay more An isolationist note crept into the arguments over disengagement.

The Iraq war sharpened Washington's scepticism but Afghanistan has deepened this rift. Nato took on its first out-of-area mission without assessing the state of public opinion in contributor countries. No member wants to undermine an alliance that has proved vital in winning the Cold War. But the existential threat is gone; in its place is only bickering about roles, money, procurement and attempts at common weapons systems. Many among the founder members now believe Nato's job is done. It bodes ill for the 60th anniversary next year.

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