Leading Article: If Nato is to be bigger, will it also be better?

How odd, it may seem at first glance, that this week's Nato summit in Madrid should be proving to be so contentious an affair. After all, the alliance has comprehensively won the great Cold War confrontation for which it was created, without firing a shot. Now, in a historic redrawing of the European security map, Poland, the Czech Republic and Hungary, three members of its former Warsaw Pact foe, are poised to take part in the biggest single enlargement of Nato in its 48-year existence. Germany, cause of so much of the continent's misery this century, is about to be entirely surrounded by countries that are bound to it in voluntary alliance. Bosnia may still smoulder, but rarely in history has the risk of major conflict in the heart of Europe appeared so remote.

Enough, surely, for profound and unequivocal rejoicing. But not so. France is squabbling with the US over the distribution of power within Nato's command structure. The exact future mission of the alliance is uncertain. Above all, expansion is in dispute: not just the number of those to be admitted, but the very wisdom of the exercise itself.

Such, in a sense, are the problems of success. Winning the Cold War is indeed a hard act to follow. But the problems also reflect confusion and contradictions that, unless resolved, could threaten the very future of the alliance.

Take expansion, the issue on which argument has been fiercest, and where the contradiction is most vivid. To reject the idea of enlargement, and thus limit Nato to its Soviet-era configuration, is to perpetuate Cold War thinking. And yet these first new admissions are taking place for the most basic of Cold War reasons: fear of Russia. Poland, the Czech Republic and Hungary admit quite openly that they are seeking membership to ensure protection from their giant neighbour to the East. And guilt at having failed central Europe, first at Munich and then at Yalta, is the prime reason the West feels it cannot deny that protection now.

"Would they die for Gdansk?" sceptics may ask of America's isolationists, in a foretaste of arguments that will certainly be heard when the US Senate debates the enlargement treaty. To which the correct reply is, would they die for Gaul, or Garmisch Partenkirchen, or even Guildford? Happily Nato's core promise, that an attack against one member is an attack against all, has never been tested in a hot war. But it is, in reality, inconceivable that America would allow the new Europe to fall victim to an aggressor.

That is not the real case against expansion. It is, first, that it draws a new division of Europe between those who are in and those who are still out; and, second, that it at best needlessly provokes Russia, and at worst provides a pretext for nationalists, xenophobes and others of an undemocratic bent to regain power in Moscow.

Both arguments have merit. If Poland, the Czech Republic and Hungary, then why not Slovenia, Romania, Bulgaria and the Baltic states? And why rub salt gratuitously into the wounds of a Russia that is in military and economic tatters, and is less of a threat to its neighbours than at any time since the 1917 revolution? It is easy, therefore, to sympathise with the impressive lobby of experts in the US and beyond which insists that enlargement is the wrong idea at the wrong time, and that the three newcomers have better things to spend their money on than fancy military hardware from the US arms industry.

Now, however, debate has been overtaken by fact. With Russia's sulking acquiescence Nato is expanding, and will expand farther in the future - perhaps, one day, to embrace Russia itself. Lost amid the quarrel over numbers is the far larger question of what the alliance is for anyway, six years after the demise of the enemy that was its raison d'etre.

The minimalist answer is that the alliance at least protects its members from each other and from themselves. The assertion holds up in part (almost certainly Greece and Turkey would have gone to war, had they not both belonged to the alliance since 1952); but only in part. Nato membership, it is claimed, is a certificate of good democracy. But it is not a certificate that anyone bothered to withdraw from the colonels who ruled Greece between 1967 and 1974. A more plausible guarantee of democracy would be membership of the European Union. Sadly, rich Western Europe has preferred to agonise over the search for a common currency, rather than throw open its doors to the aspirants from the East. But that abrogation of historic responsibility is another story.

Ever more probably, the military future of Nato lies as peacemaker and peace-enforcer. Thus far such activities have been confined to Bosnia, but with no mean success. Almost certainly this role will be extended outside Europe; indeed, under thin United Nations camouflage, it already has been, in Operation Desert Storm against Iraq.

But the line to be walked is very fine. The Gulf war had overwhelming international support. Under only slightly different circumstances, however, the alliance could be transformed into the military arm of Western liberal civilisation and its economic interests, marching into battle against the forces of Islam or Asia. At that point, the interests of the US and some or all of its European allies might well part company, and the unique transatlantic common purpose on which Nato was built might be broken for good.

Beyond expansion, that is the real challenge facing the alliance; and therein may lie the crack that leads to its ultimate fracture.