For four decades, the nations of the West trained their troops to defend against the Red Army. Now, nearly 10 years after the collapse of the Soviet Union, the issue is no longer Russian aggression against NATO. It's Russian accession to NATO.

President Vladimir Putin floated the idea last month, suggesting it was time for the Atlantic alliance to consider accepting Russia as a member of the treaty organization founded half a century ago to counter Moscow's growing power. Either NATO should stop viewing Russia as an enemy, he said, or it should disband as a geopolitical anachronism.

Others have picked up the theme. German Chancellor Gerhard Schroeder last week spoke approvingly of the idea of Russian membership in NATO, while President Bush has raised the concept in private with advisers, according to a source informed about the discussions. The idea has won public endorsement from legislative leaders ranging from House Minority Leader Richard A. Gephardt (D-Mo.) in Washington to ultranationalist State Duma member Vladimir Zhirinovsky in Moscow.

No one expects the Kremlin to be filling out a membership application anytime soon. In the short term, broaching the topic allows both sides to make political points as NATO prepares to expand farther east. According to analysts, the West is dangling the prospect of NATO membership to make it easier for Moscow to swallow admission of the Baltic republics. Putin, meanwhile, wants to underscore the contradiction in extending the alliance to Russian borders while maintaining that it is not targeted against his country.

"It's political," said Pavel Felgenhauer, a military analyst in Moscow. "Russia cannot join NATO; everybody knows that. . . . Russia's saying it wants to join NATO, NATO says anyone can apply, but of course it's all nonsense. The West just wants to be nice to Mr. Putin. He knows it won't happen."

But all the talk is fueling a provocative debate here and in the West about Russia's longer-term place in the European security structure and NATO's mission in the post-Cold War world. As expressed by some international specialists, the NATO membership discussion raises the questions of how and when Russia can be integrated into the security framework of the rest of the continent.

"One of the good things about this being on the theoretical agenda is [that] people will start talking about the big picture," said Celeste Wallander, director of the Russian program at the Center for Strategic and International Studies in Washington. In her view, the debate about NATO enlargement in the 1990s focused too narrowly on which countries would be enrolled and which would have to wait. "We didn't really have the debate about what NATO is, what it should be and what it will be. We pushed down the road the bigger debate."

During President Bill Clinton's term, NATO enlarged into former communist Eastern Europe by admitting Poland, Hungary and the Czech Republic. Bush has vowed to continue the process, saying in a recent speech in Warsaw that he could envision a NATO stretching from the Baltic Sea to the Black Sea.

Nine countries have applied for the next expansion round, including Lithuania, Latvia and Estonia, three former parts of the Soviet empire that remain especially touchy for Moscow. The 19 current NATO partners intend to decide which to accept at a November 2002 summit in Prague.

NATO spokesman Yves Brodeur said the question of Russia has not come up much. "This is not part of the actual debate at all," he said. "It's a non-issue at this point."

And Russian Defense Minister Sergei Ivanov, who plans to meet with U.S. Defense Secretary Donald H. Rumsfeld here on Monday, tried to play down the issue late last week. "Russian entry into NATO in the near future is unrealistic or improbable," he said.

But he added that Moscow favored closer cooperation with NATO against security threats in such places as the Middle East or the Balkans. "I have noticed that our Western partners do want integration in the sphere of security," he said. "We want it too." And the Kremlin appears to be encouraging discussion of the topic. A Kremlin-affiliated Internet site, www.strana.ru, posted 13 articles on possible Russian membership in NATO last week.

Putin brought up the idea at a July 18 news conference before meeting with Bush in Genoa, Italy, while discussing how to redefine the relationship between East and West. One "option is to include Russia in NATO," he said. "This also creates a single defense and security space."

Gephardt embraced the concept after visiting Russia this summer. "NATO was created to defend against common threats and reduce conflicts among its members, so what better way to prevent a new Cold War or something worse than to extend to Russia the prospect of NATO membership?" he said in an Aug. 2 speech.

Citing comments he attributed to Bush's national security adviser Condoleezza Rice, Schroeder said last week that it was "very encouraging" that the United States thought Russia could join NATO one day. While he did not specify when Rice made such comments, he seconded the sentiment. "If one puts things into a historical context, Russia's membership in NATO cannot be ruled out in the long term," he said.

Asked about Schroeder's comment, a U.S. administration official said Rice's position was that "in America, one day, one could imagine that Russia could contemplate NATO membership" -- a more cautious formulation than the German leader suggested.

As a realistic matter, Russia at this point would not meet the political and military criteria set by NATO for new members, according to a broad range of analysts. Among other things, NATO looks at political freedoms, democratic institutions and the rights of minorities. Russia's continuing war in Chechnya would give many members pause, said NATO spokesman Brodeur. Other specialists noted that Russia's military budget is not transparent and its armed forces are not truly under civilian control.

Moreover, Western officials pointed out that Russia has not eagerly embraced the potential for cooperation with NATO already offered through programs such as the Partnership for Peace. Moscow has been reluctant to join in such programs, which would put it on the same level as countries such as Albania. Except for contributing peacekeeping troops in the Balkans, Russia has largely shied away from joint exercises or training that the U.S. military has proposed.

But a Western diplomat here said that the latest comments by Putin were cause for optimism. "What I find interesting is that Putin is the one who put this back on the table," he said. "He did bring it back into the debate. We're interested now in trying to pick up where he left off."

Staff writer Steven Mufson contributed to this report from Washington.

German Chancellor Schroeder, left, said, "Russia's membership in NATO cannot be ruled out in the long term" and that Condoleezza Rice, U.S. national security adviser, shared that sentiment. A U.S. official described her view more cautiously.Russian President Vladimir Putin said including Russia in NATO would create "a single defense and security space."