It is lunchtime at Nato headquarters outside Brussels and there is a new buzz in the air. The 4,000 workers, civilian and military, mingle across the vast, sprawling canteen, excitedly debating a single question.

After years of self-doubt over its relevance in a post-Cold War world, has the 50-year-old alliance finally found a 21st-century role thanks to a resurgent Russian bear? Since the Soviet Union collapsed, Nato has struggled to reconfigure itself from a rigid defensive posture in Europe to a vast expeditionary force capable of taking on campaigns such as the war in Afghanistan.

“The main subject of conversation in the coffee bars is whether this was a tipping point and whether an alliance that has been focused on Afghanistan needs to recalibrate its view of Russia,” a senior British official at Nato headquarters said.

“We are not saying how can we punish Russia. We are saying how can we manage a Russia which has clearly got a new agenda, which says it has special rights over its near neighbours.”

Those near neighbours already in Nato are the ones leading the charge to put the Russian threat back on the map. “Now, because of Georgia, there are Nato members such as Poland, the Czech Republic and the Baltic states who are saying the alliance should stop thinking about expeditionary warfare and concentrate once again on old-style military structures to deter Russia,” a senior alliance source told The Times.

“Their plea is 'Nato come home', but we can't ditch Afghanistan to shore up Poland or the Baltic states to deter an assertive Russia.”

The division between those who still want to focus the main effort on Afghanistan and others who believe that resources should be switched back to confronting Russia's rediscovered imperialist ambitions has created turmoil within the alliance. Key to this conflict are the tough decisions to be made over who gets to join the alliance, and when.

European leaders will meet in Paris today (after the summit was moved from its original destination of Évian), to encourage closer ties with would-be Nato member Ukraine, but the alliance is deeply divided over the wisdom of allowing it and Georgia to join for fear of provoking Russia.

Britain's view is that there must be “no rowing back” from the pledge, made at the Nato summit in Bucharest in April, that Georgia and Ukraine would one day share full membership rights. This view is no longer shared by Germany, France or Italy, who question the wisdom of eastward expansion that would unnerve an unpredictable Kremlin. The push for continued enlargement is led by the United States, whose hawkish new Nato Ambassador, Kurt Volker, arrived in Brussels as the bullets were still flying in Georgia and rushed straight into an emergency meeting before even entering his office.

“I thought Nato should have issued a clear statement that day, calling for a ceasefire and withdrawal of troops, but Nato was still just waking up from summer holidays,” he said in a speech last week that highlighted his frustrations with the pace and power of the response.

He described Nato's more considered response a few days later as “a big step from that first day one week earlier” but is still angry that not enough was done to effect a Russian withdrawal.

“Co-operation needs to be based on working with a Russia that respects its neighbours ... we cannot simply go on with business as usual in our relations with Russia while Russia continues to occupy Georgian territory, destroy its infrastructure and divide a sovereign country. While the Cold War is over, threats to our democratic, transatlantic community have not gone away.”

While the United States continues to push for Georgia to be given “membership action plan status”, the crucial first step towards membership, at Nato's crucial foreign ministers meeting in December, even senior Nato advisers are suggesting that it would be more prudent to concentrate on propping up Tbilisi's democratic regime in the face of the Russian desire for regime change, rather than going down the membership route.

Nato's decision to consider the Georgian and Ukrainian applications together has not helped Tbilisi's Nato ambitions, especially as the Government in Ukraine teeters on the brink of collapse.

“It means we can't have one without the other, which has put the alliance on the spot,” an official said, “Particularly now that the Government in Kiev is split between those who want to be closer to the West and those who prefer to remain allied to Moscow.”

Under the terms of the enlargement programme no applicant country can join if it is involved in a continuing territorial conflict. This would bar Georgia from joining if Russian troops remained in the country.

Even if Georgia were given the green light it would not be entitled to make an Article Five request, under which every member state is obliged to go to the aid of another member under attack. This right is bestowed only when a country is a fully paid-up member of the alliance.

Nato's map could be redrawn northwards as well. The Georgia crisis has revived debate on Nato membership in Finland and Sweden.

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