Leading article: Nato is finally in transition, but what is it actually for?

Leaders of the North Atlantic alliance will today conclude a two-day summit that was billed by the Nato secretary-general, Anders Fogh Rasmussen, as one of the most important in its history. Held in Lisbon, it has three main agenda items: agreement on a new Strategic Concept, the war in Afghanistan, and relations with Russia, including revised plans for a missile defence system. This disparate collection of priorities can be seen either as justification for Nato's continued existence or as proof that the alliance is out of time and has lost its way.

Outwardly, Nato has changed since the Berlin Wall and communism fell. It has incorporated former East and Central European members of the Warsaw Pact and expanded – controversially – up to the Russian border. When it lent its imprimatur, and its command structures, to the war in Afghanistan, its forces were engaged in combat beyond the European theatre for the first time. But its thinking remained stubbornly the same.

It is entirely right that Nato should, belatedly, be considering exactly what, in the 21st century, it is for. Nor is it the only international organisation to have had to face new realities. But it is the one whose very nature has been most directly challenged. For 40 years, its whole reason for existing was the military threat posed by the Soviet Union and Western Europe's need for stouter defences than it could provide alone. When the division of Europe ended, so did that clear rationale.

Many of the questions to be decided at this summit should ideally have been considered in the early 1990s. In that sense, this summit is almost 20 years late. There were reasons for the delay – not least the determination of the eastern and central Europeans to join the alliance that they had seen as such an effective defence against the Soviet Union. There were the disagreements about how to handle the disintegration of Yugoslavia and the often recalcitrant stance of Russia.

Even more significant perhaps was the reorientation of policy in Washington following 9/11, which placed the threat of terrorism front and central. With Russia weak and most of Europe preoccupied with its own restructuring, many traditional concerns of the North Atlantic alliance simply slipped away. But the magnitude of the transformation that took place in the late 1980s and early 1990s surely contributed to the delay, too. It has taken a generation for these changes to be absorbed and still, perhaps, their ramifications are not fully understood.

In retrospect, this summit may be seen as finally marking the transition from Cold War to post-Cold War world. The revised plans for a Nato missile shield and the presence of the Russian President – consequences of President Obama's decision to "re-set" relations with Russia and President Medvedev's stated intention of pursuing a more Western-orientated foreign policy – bode well. But they also raise questions about Nato's purpose.

If, as sometimes appears, the problematic Afghan war is all that holds the alliance together, it is worth asking whether that war is an aberration or the shape of things to come. At best, this summit will show Nato as an alliance in transition, rather than frozen in the past. As to its future, and whether it has one as a North Atlantic defence alliance, that much bigger question will be for another day.