So, far from being an exercise in self-congratulation, the meeting is more likely to be a worried council of war - a kind of war that the Alliance has never fought, and was never intended to fight.
Nato still has much to congratulate itself about. It can rightly claim to have fulfilled all its original aims, which were well defined by its first Secretary-General, Lord Ismay, as being to keep the Americans in, the Russians out, and the Germans down. The Americans became deeply committed to the defence of Western Europe against what was perceived, rightly or wrongly, as a Soviet threat of military conquest. The Federal Republic of Germany was peacefully absorbed into a Western Europe whose unity its leaders played a major role in creating. Whether the Soviet threat was exaggerated or not, American protection enabled Western Europe to recover its nerve politically and to create an economy wealthy beyond any historical precedent. The price paid was the abandonment of Central and Eastern Europe to Soviet control; but, even there, the stability created by the Iron Curtain was preferable to a third World War.
Now the Cold War is over, and Nato can take much of the credit; not just for keeping it cold, but for bringing it to an end at the earliest possible moment with a remarkable absence of bitterness on either side. "The German Problem" is now history. The Soviet Union no longer exists. For the past 10 years, therefore, the Allies have been debating how to reconstruct the Alliance to meet an entirely new situation.
The idea of dismantling Nato altogether has occasionally been raised, usually on the other side of the Atlantic, but never taken seriously. Nato is not just a military alliance; it is an institution, with a large military and civil bureaucracy whose members have a vested interest in its survival. The armed forces of the smaller European powers make no sense except within the Alliance framework, while the Germans still see the Alliance as a necessary legitimisation of their own military power. Most of them would agree that the renationalisation of European armed forces would be a foolish, if not dangerous, retrograde step; Nato provides for Europeans a "security community" that guarantees them against threats not only from outside but also from one another. Also, it does so on the cheap: the Americans still provide all the expensive hardware and infrastructure, the cost of which would severely erode even the wealthiest European economy.
The American interest in Nato is more subtle. The United States still feels itself deeply committed to the preservation of world order. Its enemies may see this order as a hegemony based on economic motives; its friends share the belief that it is a genuine and admirable moral commitment; but in any case it is a burden that the Americans are reluctant to carry on their own. They need to believe that when they do intervene to protect or restore that order, they are doing so on behalf of the "international community".
Initially, they hoped that this community might be embodied in the United Nations. For a few golden years at the end of the Cold War, when the Russians were compliant supplicants if not partners for the United States, and the Chinese were post-Tianenmen pariahs, perhaps it really was. But this is no longer the case; the UN is seen as being too divided in its councils to provide a supportive consensus for American policy. Now the "international community" is, for the United States, its like-minded partners in Nato, whose co-operation it seeks in order to legitimise its actions. For Washington the military importance of Nato may have dwindled with the end of the Cold War, but the political significance of the Alliance is greater than ever. In consequence the Americans, so far from wishing to wind up Nato, have sought to expand both its membership and its field of action beyond the strict defensive limits of Europe and North America that are laid down in the text of the treaty. "Out of area," as one American congressman brutally put it, "or out of business".
Throughout the first 50 years of its existence, the members of the Alliance quarrelled bitterly over two major issues: the requirements of nuclear deterrence and the desirability of political detente. The literature dealing with these subjects was as voluminous, and is now as irrelevant, as theological tomes debating the Real Presence. Recently Nato has been wrestling with three very different problems: the expansion of membership; the assumption of responsibilities "out of area"; and the development within the Alliance of a distinct European element, a "European Security and Defence Identity", to match the growing political and economic unity of the continent. So the Allies had plenty on their minds even before they became involved in the third Balkan war.
Let us take the last issue first. In principle, the Americans have been supportive of a distinctive European contribution to the Alliance. Ever since the days of John F Kennedy they have been urging the Europeans to combine so as to take their fair share of the burden of their own defence, and to create a "second pillar" for Nato. More recently they have seen the additional advantage of a European force that might be used in situations where domestic public opinion would not permit the commitment of US troops, but for which the US might provide the necessary logistical support. Among Europeans, the French have been most eager to see this develop; a separate command of this kind would provide them with the opportunity to exert military leadership in Europe and counterbalance the Anglo- Saxon dominance they so much resent. The Maastricht Treaty, with its commitment to a common European defence and foreign policy, provided the catalyst for a renewed initiative, but one which both the Americans and the British have regarded with deep suspicion. So for a decade the Allies have been wrangling over the possibility of creating what they term "combined joint task forces", which would be "separable but not separate" (an endlessly repeated mantra), producing mountains of paper, clouds of new acronyms, and a general atmosphere in Brussels of cynical confusion. Recently Mr Blair has tried to show what a good European he is by sinking his differences with the French, but "the European defence identity" remains a concept that has yet to be clothed with flesh.
Secondly there is Nato expansion - an issue over which the Alliance is still deeply divided, though not on national lines. On the one side are those who see Nato primarily as a mutually supportive alliance of democracies to which any like-minded state is entitled to belong, especially those who suffered for so long under Soviet oppression. On the other are those concerned with the military and political effectiveness of an Alliance that can barely function with 16 members, and for whom More will inevitably mean Worse. They are joined by Realpolitikers for whom good relations with Russia take precedence over sentimental feelings towards small states in Central Europe who are more likely to prove military liabilities rather than assets, and in any case have nowhere else to go.
The decision to admit Poland, Hungary and the Czech Republic was taken in Washington largely in response to ethnic domestic pressures, and on the urging of a Czech-born Secretary of State. These three states can certainly claim to have been arbitrarily separated from a West to which they culturally belong, but their admission opens a Pandora's box; especially since it has been officially declared that they will not be the last. Who next? Hungary's admission makes little strategic sense unless it is joined by its neighbour Slovenia. The French will not rest content without the admission of their traditional ally Romania, and can Bulgaria then be left isolated between Romania and Turkey? And what about the Baltic states - the only ones who might still need protection against the Russians? For the unfortunate military staffs whose task it is to integrate the wildly differing armed forces of these countries into the Alliance's structure and plan their integrated defence, the prospect is a nightmare.
If it could be accepted that the collapse of the Soviet Union implies that Nato need no longer be regarded as a serious military alliance, but simply as a "security community" of friendly and like-minded nations, a sort of inner circle of the Partnership for Peace, then no problem arises. Membership could then be mended almost indefinitely, and the swords could be beaten, if not into ploughshares, then into bayonets for purely ceremonial use. But that is not the way the United States sees it, which brings us to the final problem: "Out of Area".
"More means Worse" not just for military planners but also for political decision-makers. Even over issues narrowly concerned with area defence, it has always been a herculean task to get agreement between Copenhagen, Lisbon, Rome, Oslo, Paris, London and Washington, not to mention Athens and Ankara. To toss Warsaw, Prague, Budapest, Bucharest and possibly Vilnius into this cauldron can result only in political boil-and-bubble with a minimum of positive result. Any suggestion of supporting United States intervention anywhere outside the European area, unless it bears the imprimatur of the United Nations or directly threatens common European interests, is likely to provoke strong domestic dissent in every member of the Alliance. The more widely membership of the Alliance is extended, the more likely is this to be the case.
The exception, of course, is the Balkans - that squalling fosterchild deposited on the lap of the European Powers by the dying Ottoman Empire a century and a half ago. The Balkans are hardly "out of area" for Greece, Turkey or Hungary, and would not be for Slovenia, Bulgaria or Romania were they to join. The region has always played a large if unwelcome role in the foreign policies of France, Britain and Italy, it is of special interest to Russia, and in consequence to Realpolitikers in the United Slates. Nato has, in fact, inherited the mantle of the European Powers which repeatedly found themselves compelled collectively to intervene in the region between 1875 and 1914, when their involvement escalated into a global conflict whose consequences pushed the region on to the back-burner for the rest of the century.
Now it is back on the agenda. The European members of the Alliance are involved, not as loyal allies supporting and legitimising an American- inspired intervention, but as the legatees of the old Concert of Europe and thus principal actors in their own right. The United States is involved, not just to pull European chestnuts out of the fire for them, but also in a moral crusade against evil - and this is a point as constantly and skilfully made to them by the British Prime Minister today as it was 60 years ago - of the kind that they believed they were waging in both World Wars. Suddenly the Alliance has found a purpose to justify its existence to the most sceptical of its critics.
It is still far too soon to pronounce judgement on the effectiveness of Nato intervention in Kosovo and its consequences for the Alliance. Nato will probably "win", but it could be at a cost that so embitters relations between the Allies as to make future co-operation almost impossible. More probably, the difficulties and hazards of the operation will draw them closer together. But almost certainly the result will be a lasting commitment to policing the region, of a kind that the Powers before 1914 did their very best to avoid.
The Balkans may or may not be "out of area", but either way they are likely to keep the Alliance "in business" for a very long time to come.
The author was Regius Professor of modern history at Oxford University from 1980 to 1989