LONDON -- The June 28-29 summit meeting of the North Atlantic Treaty Organization in Istanbul was a sour affair. The so-called allies within NATO could not agree on how to help with reconstruction in Iraq and ended up merely offering to do some training of Iraqi personnel, but not much more.

Bad feelings intensified when U.S. President George W. Bush chose the occasion to urge the European Union to speed up their admittance of Turkey to the club -- an issue on which the French, in particular, have grave doubts. French President Jacques Chirac promptly told Bush and the Americans to mind their own business.

But the unhappy gathering gave rise to an even more fundamental question, namely, whether NATO itself has a future. With the Cold War long since won, with Russia now considered a security partner and most of the former Communist satellites now on the Western side, and with the main threats to global security arising far outside the European theater, an increasing number of voices -- both sides of the Atlantic -- are asking "Why do we need NATO at all?"

Up to now the conventional answer to such questions has been that America has all the military muscle, all the satellite networks, the heavy air transport, the technological wizardry and so on to make it a necessary player in any but the most minor policing operations. At the same time there has been the sentimental element left over from World War II -- that the Atlantic partnership must at all costs be maintained.

But suddenly people in Europe are querying whether the vast American defense machine is really the right or best instrument to ensure European security and meet the new and subtle threats of the global terrorist age.

Why, it is asked, does European security need this heavy American kit? And is U.S. intelligence and satellite-based information worth having after the disastrous performance of the intelligence agencies in recent years and months? Has not the time come to challenge the key assumption of Europeans ever since 1945 -- that in the last resort their security will be protected by American power and dominance. In other words, will everyone always be safe and cozy under the mighty American nuclear umbrella?

This questioning goes far beyond the familiar differences about the invasion of Iraq. It raises fundamental issues about the nature of today's security threats, about policing the world and how it should be done and by whom. As this debate develops further it is going to place the British, in particular, in a profound quandary.

Of all the European powers Britain has been in security terms far the closest to the United States for the past 50 years and has placed the greatest reliance on the NATO structure under American leadership.

It is of course true that the British have their own nuclear deterrent, as do the French. But there is a major difference. The French force de frappe may be expensive and dated, but it is truly French based. The British Trident system depends on American technology, as will any replacement. A break with the U.S., or a re-alignment from semi-dependence on the Americans would require Britain either to move toward its own nuclear-weapons capability or drop out of the nuclear weapons league altogether.

Is that so unthinkable? Amid current world conditions, the British are highly unlikely to want to give up their nuclear power status altogether, but it does seem odd that when so many nations round the world have acquired nuclear weapons, for better or worse, (China, India, Pakistan, Israel for example) on their own, Britain, now said to be Europe's richest country, should still lean so heavily on its American nuclear linkages.

In a way the dilemma is analogous to that facing Japan, although the two countries start from different positions. But in both cases the underlying new question is how far to rely on the U.S., and will remaining under the American wing in fact provide the security against modern threats that a democratic and open society needs.

Or must both countries now depend much more on their own resources to protect their national safety and interests, and on close alliances with friendly nations in their neighborhood?

Optimists may think these awkward questions can be ducked for many years to come. But if the Europeans are concluding that the mighty protection of the U.S. is no longer all that desirable and effective, and if the Americans are concluding that they would rather not continue shouldering the burden of Europe's defense and security, then the whole game changes. The U.S. remains a friend and ally, of course, but no longer the kingpin, either in NATO or anywhere else.

How curious it is that just when the U.S. seemed to have become the world's ultimate and only superpower, with comparisons being made with the imperial might and reach of Rome, it should suddenly be seen by the rest of the world as not half so high and mighty and not necessarily always the best protector to have. But then history is full of huge ironies, and we may now be witnessing one of them.