The shift, signalled by Yevgeny Primakov, the Russian Foreign Minister, in yesterday's Berlin meeting with his Nato counterparts in Berlin, leaves the door ajar for some former Warsaw Pact member states to join Nato, while consigning other remnants of the defunct Eastern Bloc to a security limbo.
Western diplomats say that Mr Primakov told the private meeting he had "no objections to expansion, as long as it only has political consequences".
Mr Primakov was said to have described the transfer of Nato infrastructure towards Russia's western borderlands as "unacceptable".
The alliance, oozing reassurance, was at pains to soothe Moscow's concerns. "He has nothing to fear," said a Nato source. "There will not be massive movements of Nato troops or infrastructure towards the Russian frontier."
Although no concrete deal was struck - a decision about enlargement had already been tactfully postponed until after the Russian elections - Western officials concede that the formula may initially exclude Russia's immediate neighbours from Nato membership.
The Baltic republics, and perhaps even Poland, which has a short border with Russia around the Kaliningrad enclave, could thus fall victim to the rapprochement between East and West.
"This [Primakov statement] shows a significant shift from the previous stance, which entirely rejected expansion," said Laszlo Kovacs, the Foreign Minister of Hungary. "The main thing is that the door remains open to seek further compromises."
Hungary is one of the aspiring members likely to be admitted in the first batch of new Nato members.
The other immediate beneficiaries of Moscow's apparent volte-face are likely to be the Czech Republic and, perhaps, Poland.
Hungary, which already provides the logistical base for thousands of American soldiers serving in I-For in Bosnia, is confident the present military relationship will not, in the words of a Nato official, "become a one-night stand" after I-For's mandate expires in December.
Negotiations about accession will begin next year. Although the East Europeans had hoped for an earlier time-table, Helmut Kohl, the German Chancellor, called yesterday for a six-month delay, in order to keep the delicate discussions out of the maelstrom of domestic politics in the two key countries.
Nato enlargement is a factor in both the Russian and the United States presidential elections and, as Chancellor Kohl noted, "wisdom and circumspection" are scarcities at such times.
Countries that do not make the grade will be offered closer ties with Nato under its Partnership for Peace programme, which is to be upgraded to "more than a halfway house". What that will mean in practice will be determined at the next Nato meeting, in December, by which time it is expected Russia's intentions will have become clearer.
The two-day event concluded yesterday was the most successful Nato meeting in years, due largely to the rapport established by the current Russian administration and Western leaders anxious to avoid upsetting President Boris Yeltsin's re-election campaign.
Instead of a barrage of anti-Western rhetoric, Mr Primakov wrong-footed Nato diplomats by bombarding them with praise. "We believe that Nato is playing an important role. It is adapting to new realities," he said.
Russia was particularly pleased with Nato's final communique, which for the first time recognised it as a real partner.
Mr Primakov returned the compliment by advocating more institutional ties, with more regular meetings between the alliance and Moscow's representatives.
Russia, he declared, was ready to start talks on an "enhanced relationship" with the alliance, and he expressedinterest in Nato's plan to develop a new anti- missile system.