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, NATO is riven by 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.

"We're a bunch of stray cats going forward and no one has brought us together," said Ronald Asmus, who works in the Brussels office of the German Marshall Fund of the United States.

At this weekend's 60th anniversary summit, which President Barack Obama is to attend as part of his first official trip to Europe, NATO members are expected to reaffirm their vow of mutual defense. But what that pledge means in a changing world has become a source of intense debate—a debate made hotter by a rising, muscle-flexing Russia.

Russia'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. Until then, Eastern European and Baltic states had gone along with NATO's shift in the last 20 years to missions outside its 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. But Russia's push into Georgia caused its neighbors to again look over their shoulders.

"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.

As for Afghanistan, that war is a source of considerable friction, even among several of NATO's longtime partners. Obama has promised a "surge" in U.S. troops there but has had trouble persuading countries such as Germany to boost troop levels and allow those soldiers to engage in combat.

Expanding NATO's mandate beyond traditional military endeavors, to include matters such as cybersecurity and energy security, is just as contentious. "What NATO does is quite good, and they should focus on that—crisis management, the military side of things," said Sven Biscop, a security expert at the Egmont, the Royal Institute for International Relations, in Brussels. "Fanning out into the other fields goes beyond NATO's expertise."