When leaders of NATO’s 28 member nations gather at a resort in the lush hills of Wales on Thursday, they will set the course for a military alliance that has long seemed adrift and archaic but has lately been given new vitality by an old foe: Russia.

Pushed to the point of irrelevance by the peace in Europe that it helped to forge, NATO had until recently been casting about for a mission. It found one this year with the dismemberment of Ukraine.

But NATO’s leaders will be under pressure this week to prove that the world’s most powerful military alliance is equal to the new challenge and can still deter Russia as it did during the Cold War.

The evidence in recent months has suggested otherwise, with Russian troops and weapons pouring into Ukraine despite repeated NATO condemnations and warnings. Analysts say that NATO is unlikely to make any bold moves this week that could change the trajectory of the separatist conflict in Ukraine, particularly following news of apparent diplomatic progress on Wednesday. Ukraine is a NATO partner, not a member, and the alliance has repeatedly made clear that it is unwilling to deploy military forces or send arms.

“There’s a delicate question about how far you go, as a NATO alliance, before you find yourself in a war with Russia,” said Lukasz Kulesa, research director at the European Leadership Network and a former top official in Poland’s National Security Bureau. “The NATO leaders will want to play it safe.”

President Obama called Russia's actions "a brazen assault on the territorial integrity of Ukraine" during a speech at the Nordea Concert Hall in Tallinn, Estonia, on Wednesday. (WhiteHouse.gov)

But that does not mean the summit will be any less critical for the organization. Kulesa said NATO’s top objective will be to reassure allies in Eastern Europe that fear they could be next in Russia’s crosshairs.

“The member states will want to highlight the difference between, say, Estonia and Ukraine,” Kulesa said, citing a NATO member that President Obama visited Wednesday.

Obama, who met in Estonia with leaders from the three Baltic states, said the alliance stands ready to defend them against possible Russian aggression.

“You lost your independence before,” he said. “With NATO, you will never lose it again.”

Obama said that progress toward a democratic, free Europe has been endangered by the crisis in Ukraine.

“As we gather here today, however, this vision is threatened by Russia’s aggression against Ukraine,” he told a group of mostly students during a speech at the Nordea Concert Hall in Tallinn, Estonia’s capital. “Reaching back to the days of the czars — trying to reclaim lands ‘lost’ in the 19th century — is not the way to secure Russia’s greatness. It only shows that unrestrained nationalism is the last refuge for those who cannot deliver real progress and opportunity to their own people at home.”

Central to the reassurance strategy is a plan that NATO leaders expect to adopt this week for a vanguard force of about 4,000 troops that could be deployed within hours anywhere in the world, but especially along Europe’s eastern border with Russia. NATO would stockpile more munitions, supplies and equipment at bases in Poland, the Baltics and Romania to enable the force to travel light.

Although NATO leaders have touted the plan, officials said details will be sorted out after the summit. According to U.S. officials, the alliance is considering whether to establish a command-and-control hub in Szczecin, Poland — a port city on the Baltic Sea near the German border. But the precise makeup of the force remains unclear, as is the question of where it would be ordinarily stationed.

NATO has tried to organize
rapid-reaction forces in the past, with meager results. It first announced it would create a NATO Response Force in 2002, with as many as 13,000 troops. But it took two years to get the unit up and running. Even today, the force needs about 30 days to mobilize. Until this year, it had deployed only once, in 2005, to provide earthquake relief to Pakistan.

The newly designed force would be part of that larger unit. But analysts questioned whether it would be any more effective.

“In theory, it sounds great: 4,000 troops ready to deploy on a moment’s notice,” said Julianne Smith, a senior fellow at the Washington-based Center for a New American Security and a former deputy national security adviser to Vice President Biden. “But one has to ask: Will this be any different?”

She questioned whether the advance force would be trained or equipped to deal with unconventional threats that are more likely to emanate from Russia than a full-blown ground invasion. “What NATO isn’t grappling with is, what about little green men? What about a cyberattack? What about a propaganda war?” Smith said. “That’s what NATO can’t figure out how to tackle.”

Although the quick-reaction force is expected to be the centerpiece of the plan presented at the summit, NATO members already have stepped up their contributions to joint air policing missions over the Baltic states.

Latvian Foreign Minister Edgars Rinkevics, whose nation is considered among the most vulnerable to possible Russian advances, said in an interview that the moves expected this week were “steps in the right direction,” and he praised NATO allies for waking up to the threat Russia poses after years of trying to turn the nation into a partner.

“The Baltics have been called hysterical. But current events have proven we are not hysterical. We have been prudent,” he said.

Still, Rinkevics said NATO has not been willing to go as far as Latvia would like in shoring up its eastern flank. There are no plans, for instance, to permanently station foreign troops in the Baltics, and Rinkevics said he understands Ukrainian frustration over the alliance’s not doing more even as evidence of direct Russian involvement in the separatist conflict has grown.

“The world is not like it was a year ago. Sometimes it’s difficult for decision-makers to see that,” he said.

Many NATO members in Europe have been deeply reluctant to challenge Russia — both for fear of spurring a wider conflict and because of economic problems at home that could be exacerbated by a confrontation.

“Putin is laying down a challenge not just to the security order in Europe, but to the whole international order,” Nick Witney, who was the first chief executive of the European Defense Agency, said of Russian President Vladi­mir Putin. “But if you really get serious about isolating Russia, then you could have a massive economic crisis in Europe.”

Alexander Vershbow, NATO’s deputy secretary general, said in a speech in Wales on Tuesday that Moscow had taken clear note of the alliance’s backsliding on defense spending. Overall, NATO military budgets have shrunk by 20 percent over the past five years, while Russia’s budget has risen by half.

“To the Kremlin, which measures its global standing in military might, NATO appears weak,” said Vershbow, an American who previously served as U.S. ambassador to Russia and also held a senior post at the Pentagon. “Our job now is to correct that perception — to be united in word and deed, and to increase spending on defense.”

Whitlock reported from Washington. Karla Adam in London contributed to this report.