Other countries, including the United States, call for a NATO doing out-of-area or expeditionary missions and combating terrorism, cyber attacks and other nontraditional threats.

Meanwhile, France, Germany and others wish to see a streamlined and more effective NATO in defense of Europe with less out-of-area missions yet more cooperation with Russia and enhanced consultations with the United Nations and other organizations.

"So what is NATO for? That's the third and I guess most enduring challenge," Vaisse said.

U.S. interest declines

While Washington continues to see Europe as its most important economic partner, its interest in NATO has declined recently.

With no credible security threat on the horizon, Europeans spent less on defense. In contrast, U.S. military power and spending are expanding as ever, especially over the 1980s and 1990s, greatly enhancing the ability to act on its own. As U.S. decision-makers are preoccupied with events in Iraq and Afghanistan, they have become much less interested in NATO.

Vaisse observed that even though Article 5 was invoked for the first time after the 9/11 attacks, the Afghanistan mission was done pretty much by the United States on its own with a few allies like Britain and France sending in special forces in 2002, and thus was "basically not done in alliance with NATO members."

NATO remains a military alliance, but the word "military" can have many different meanings, he said.

It can be a defense alliance against missile proliferation, or a coalition supposed to go around the world and do things like it does in Afghanistan. Different role asks for different input of resources. In the case of an expeditionary role, "I think Afghanistan has severely reduced their appetite for doing out-of-area missions."

Vision "not enough"

Steven Pifer, another Brookings expert, foresees a compromise at the upcoming Lisbon summit on different visions, with NATO possibly taking some out-of-area operations but not one as largest as in Afghanistan.

When they gather in Lisbon on Friday and Saturday, he said, NATO leaders are expected to have Afghanistan, the new strategic concept, nuclear deterrent, missile defense and defense spending as well as reform and transformation of the alliance on their top agenda, with a goal of invigorating it for the challenges of the 21st century.

The new strategic concept will map out policy guidelines for the military coalition in the decade to come, according to the expert.

"But vision is not enough, we also need to implement that vision. So we have proposed and hope to have accepted a set of capabilities that the alliance, in a time of dwindling resources, will decide it must fund," U.S. ambassador to NATO Ivo Daalder, told reporters Tuesday at a White House press briefing on the upcoming summit.