Leading Article: Today NATO grows bigger. Tomorrow it must be Europe

THESE ARE difficult times for Nato, as it searches for a new doctrine, even a new raison d'etre, now that the Cold War that was the reason for its very existence is over, and the Soviet Union is no more. But we can all wholeheartedly rejoice at yesterday's ceremony in Independence, Missouri, the birthplace of president Harry Truman, under whom the alliance was created exactly 50 years ago.

The formal accession to the alliance of Poland, the Czech Republic and Hungary belatedly corrects one of the great injustices of modern European history. For these "lands in the middle", trapped between Germany and Russia, this has been a dislocating and terrible century; of invasion, war, occupation and - for most of the last half of it - subjection to the alien and disastrous ideology of Communism. All three of them are now anchored where they belong, in Europe's geopolitical mainstream, as equal members of the Atlantic community. A Polish politician has described the event as the most important moment for his country in 1,000 years. He was hardly exaggerating.

But what now? Nine other countries want to join Nato, ranging from the Baltic countries in the north to Romania, Slovenia and Bulgaria in the south. The "outs" seek the cachet that goes with membership of any successful club. They are convinced, too, that Nato membership offers a fast track into the European Union. Above all, however, and like yesterday's three entrants, they see Nato as the one cast-iron guarantee of protection that they can have against any future threat from Russia.

There is, none the less, no hurry. Not long ago, the fashionable fear was that Nato expansion would draw another dividing line through Europe, to the east of the former Iron Curtain. But nothing of the sort has happened. In fact, the very gravitational pull of the alliance has made the "outs" feel safer, prodding them into settling disputes that, in an earlier age, could have led to conflict. Further blurring distinctions is the alliance's eminently successful Partnership for Peace programme, strengthening co-operation between Nato and its non-member neighbours.

However grudgingly, Russia has acknowledged that the inclusion of the first three Warsaw Pact members was inevitable. But we should take Moscow at its word when it says that it will not tolerate membership by the three Baltic states, or any other former Soviet republics. The rhetoric may be mainly bluster, and have less to do with legitimate strategic considerations than Russia's eternal paranoia about its security. Today, as it casts around for a new "strategic concept", Nato is debating whether to extend its area of operations further afield, into the Middle East and beyond.

But, in the long run, the greatest challenge will be to find a lasting accommodation with its great eastern neighbour. Currently, Russia is no threat, but who is to say that it will remain so? Scenting new contracts, the powerful US defence industry is pressing hard for Nato enlargement. But the alliance wisely prefers to use the lull to create a new relationship with Moscow, rather than indulge in provocative gestures that could bring to power a more nationalistic, anti-Western regime. No new invitations will be issued at the alliance's birthday summit in Washington next month.

Instead, the onus is on Europe. Now, it is unfair to criticise the EU for the slow pace of negotiation with candidate members from the former Eastern bloc. Joining a military alliance is a simple and much less expensive matter than entering a far more advanced economic bloc, with all the hideously complicated negotiations that that entails. But the momentum for EU expansion must not be allowed to flag - nor the efforts that are under way, led by Britain and France, to endow the Union with a common foreign and security policy worthy of the name.

For Moscow, Europe is a far less threatening entity than Nato. But a broad and strong EU, capable of tending to its own defence, would offer a guarantee no less tempting than the one provided by Nato today.