BRUSSELS — After 60 years of a solid but sometimes stormy marriage, the countries of the world's most powerful military alliance plan to renew their vows of mutual support and protection this weekend. But don't expect a second honeymoon.
As it prepares to enter its seventh decade, the North Atlantic Treaty Organization is riven by profound disagreements over its role in a world that combines new threats with the specter of an old one.
Many critics see an alliance adrift, one that is fighting an increasingly unpopular war in Afghanistan while neglecting challenges closer to home, such as a newly resurgent Russia. But even as some of the 26 members -- rising to 28 as of this week -- advocate going "back to basics," others insist that to remain relevant in the 21st century, NATO must branch out to combat new threats to transatlantic security, from climate change to the vulnerability of cyberspace.
If NATO cannot heal its internal rifts, it risks the obsolescence of a storied alliance that guaranteed free Europe's security and faced down the Soviet Union without firing a shot.
"We're a bunch of stray cats going forward, and no one has brought us together," said Ronald D. Asmus, who works in the Brussels office of the German Marshall Fund of the United States. "Basically we don't have a consensus in NATO today on what the priorities should be."
At this weekend's 60th anniversary summit, which President Obama is to attend as part of his first official visit to Europe, NATO members are expected to reaffirm their vow of mutual defense, the bedrock of the alliance's existence. The defining principle of "one for all and all for one" was laid out in Article 5 of the organization's founding treaty in 1949, soon after the onset of the Cold War.
But what that pledge means in a changing world has become a source of intense debate between old friends, and between traditional and more recent allies -- a debate made hotter by a rising, muscle-flexing Russia.
Moscow's invasion last year of Georgia, an aspiring NATO member, was a game-changer, particularly for the former satellite states of the Soviet Union that have joined the alliance.
Up till then, Eastern European and Baltic states had gone along with NATO's shift in the last 20 years to missions outside its historical sphere of operation, including the Afghan war and the patrolling of shipping lanes in the Mediterranean Sea. "Out of area or out of business" was NATO's new mantra, reflecting post-Cold War geopolitical realities.
But Russia's push into Georgia caused its neighbors to look nervously over their shoulders again.
"What Georgia did was destroy the perception that the European continent was safe once and for all, and that war had become unthinkable," Asmus said. "You have those saying, 'Hey, it's not Afghanistan, but what about us, what about our security on the continent?' "
Former East Bloc countries such as Poland and ex-Soviet republics such as the Baltic state Lithuania are now lobbying for a vigorous commitment to mutual defense in the traditional sense of territorial protection, a throwback to the days when Article 5 was a clear call to keep Soviet boots off Western European soil.
These nations want that reaffirmation to be incorporated into a declaration of security that will be issued at the summit. They also want the alliance's military command to devise stronger contingency plans for their defense.
But that position puts them at odds with allies such as Germany and Italy, which are eager to maintain friendly relations with Russia and fear that any emphasis on Article 5 would antagonize the Kremlin and escalate tensions.
Influential voices, many of them in the U.S., also argue the alliance cannot just go back to old-time basics if it hopes to remain relevant. Instead, it must redefine security to take into account threats unknown only a decade ago: terrorist strikes on a large scale, perhaps with nuclear or biological weapons; hackers who try to disrupt vital computer networks; attacks on oil and gas pipelines; the melting of the polar icecaps and a potential scramble for natural resources there.
In the eyes of Washington and London, the battle in Afghanistan against the Taliban and Islamic extremism, albeit far away, is crucial to the domestic security of NATO countries. The alliance never invoked Article 5 during the Cold War, but did so for the first time after the Sept. 11 attacks.
"The main threat to European and American citizens emanates from turmoil and terrorism in Afghanistan and the tribal areas of Pakistan," declared a much-discussed report on NATO released in February by a group of American think tanks. "Afghanistan has become a crucible for the alliance. NATO's credibility is on the line."