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Russia's ongoing confrontation with the West has ignited debate inside and outside the U.S.-led NATO alliance about what its responsibilities are, and how much of its time and effort should be spent to prepare for and if necessary counter Russian President Vladimir Putin's military ambitions.

Ian Lesser, senior director for foreign and security policy at the German Marshall Fund of the United States, said NATO must remain mindful of other modern security challenges, including cyberterrorism, threats to energy supplies and armed Islamic extremism.

But he predicted the trans-Atlantic alliance's focus will shift dramatically because of what he termed the biggest game changer in European security and defense environment in 20 years: Russia's armed aggression in Crimea and the Kremlin's continuing military pressure on Ukraine.

"Today we have a situation in which Russia and especially the Russian leadership is highly unpredictable," Lesser said. "There is something about the current crisis that suggests Russia is a rogue state, with all that would imply for deterrence and reassurance of allies."

As the alliance ponders how to react in Europe after years of fielding operations in the Middle East, Africa and Afghanistan, one of NATO's top commanders told The Associated Press that Russia's demonstrated ability to swiftly mobilize large numbers of troops in so-called snap exercises and Moscow's uncertain intentions have forced a rethink of NATO's capacity to respond and the deployment of its forces.

"What I am thinking about now is, is NATO correctly positioned and is it at the right state of responsiveness?" U.S. Air Force Gen. Philip Breedlove, NATO's supreme commander in Europe, said in a recent interview. "If we expect that Mr. Putin is going to be in charge of Russia for many years, if we are going to see this kind of exercise behavior in the future, are we prepared to react to the next snap exercise that goes across a border to impose its will on another sovereign nation in a different part of Europe? That's what I've been doing a lot of thinking about."

Already, NATO has reinforced its Baltic air patrols, put AWACS surveillance planes in the skies over Poland and Romania, dispatched warships to the Baltic and Black seas and sent 600 U.S. Army paratroopers to Poland and the Baltic states on temporary deployment.

Discussions are under way on longer-term measures, and how NATO must reposition, retool and otherwise react to the new challenge from Moscow will be the most pressing question on the agenda when President Barack Obama and leaders of the alliance's 27 other member nations gather for a summit in Wales this September.

During a visit to Canada this week, Breedlove said he wants the political leaders to think about permanently stationing alliance forces in Eastern Europe to reinforce local defense capabilities.

"I think this is something that we have to consider, and we will tee this up for discussion through the leaderships of our nations and see where that leads," Breedlove said Wednesday in Ottawa.

If American fighting men and women are part of a new NATO mix in Eastern Europe, it could mean a halt, or even reversal, of the drastic U.S. military drawdown in Europe that began in the early 1990s as tensions between the West and Soviet Union ebbed.

From a peak of more than 420,000 uniformed personnel at the height of the Cold War, the U.S. presence in Europe has dropped to around 68,000 active-duty men and women of all branches, according to figures provided this week by the U.S. European Command.

There once had been 20 American armored divisions in Germany — around 6,000 tanks. A year ago, the last 22 U.S. main battle tanks in Europe were shipped home.

Ironically, the new challenge from Russia comes as some inside and outside NATO were wondering what the alliance was going to do once the biggest operation in its history, combating Islamic extremism in Afghanistan and helping foster political and social stability there, is due to come to an end this December.

For some analysts, the alliance's decision to take on military and stabilization tasks in such faraway places distracted it from its chief responsibility: keeping its own members safe.

"I think NATO drifted away from its core mission," Michael E. Brown, dean of the Elliott School of International Affairs at George Washington University, said. "It took on an array of global missions that proved to be very challenging for it. Now it's being forced to do what it should have been doing all along, and that's deterrence."

The last time the leaders of NATO nations met, in May 2012 in Chicago, they expressed hope of forging a "true strategic partnership between NATO and Russia." Russia's use of force to achieve the unilateral annexation in Crimea, its continuing military intimidation of Ukraine and Moscow's alleged interference in that country's ethnically restive east have dispelled those rosy dreams.

Putin's government, NATO officials say, is now acting more like an adversary than a partner, and poses the greatest threat to European security since the Soviet Union's collapse.

Asked this week by the AP whether the alliance has come full circle and must make dealing with Russia its No. 1 priority again, NATO Secretary General Anders Fogh Rasmussen replied, "Effective defense and protection of our allies remains a core task for NATO. It's been the essence of our alliance since it was established 65 years ago."

But Fogh Rasmussen immediately added that the military alliance formed in the uncertain early years of the Cold War has given itself important additional responsibilities since the demise of the Soviet bloc. Those may have nothing to do with the Russians.

"Another task is to be able to participate in international crisis management, and we have also defined cooperative security as one of our core tasks," Fogh Rasmussen said. "And we will continue to carry out those tasks as well."

Ivo Daalder, the U.S. permanent representative to NATO in 2009-2013, said in an interview that NATO simply can't go back to being obsessed with Russia all the time because of other global security challenges, including the Arab Spring.

Daalder also said today's Russia isn't the Soviet Union — it doesn't have the global ideological reach, or even the same military capability.

"Putin is not the organizing principle of our foreign and security policy, and never will be," Daalder said. "He's not important enough. He's not strong enough."

One important variable that's still unknown is whether many European allies are willing to increase their defense spending. Fogh Rasmussen has called Russian conduct toward Ukraine a "wake-up call" that means it is time for Europe to end years of skimping on military expenditures. In the same timespan that Russia has boosted defense outlays by 30 percent, some NATO countries have cut theirs by 40 percent, he has complained.

On Friday, Fogh Rasmussen said during a speech in Tallinn, Estonia, that Latvia, Lithuania and Romania have now joined Estonia in committing themselves to increased defense spending.

"And I'm confident that other allies will do so too," the NATO secretary general said. "Because defense matters. Security is precious. Freedom is priceless — and it doesn't come for free."