President Clinton is writing letters to Boris Yeltsin, trying to convince him that NATO's expansion to the east will not threaten a "cold peace."

President Clinton's missives highlight the fundamental ambiguity in NATO's role in a post-Cold War world. The purpose of the Atlantic alliance has always been to prevent Soviet domination of the European continent, but now that the Soviet threat is gone, has NATO's time passed?

While voices in America call to bring our troops home, and many Europeans, particularly the French, are calling on Europe to take responsibility for its own defense, Europe still remains a region vital for the United States' security and long-run prosperity.

Our economic and cultural ties ensure that, despite noticeable differences--witness Fidel Castro's warm reception in Paris and Gerry Adams' fundraising efforts in Washington--Western Europe still houses our staunchest allies. As the Gulf War proved, the Western allies must be ready to handle "out-of-area" problems that will arise to challenge this new world order.

While the end of the Cold War will bring needed cutbacks in defense spending, it does not mean that the U.S. should abandon its role in European defense. As the Gulf War showed, the U.S. is still the superpower backbone of any alliance actions.

Further, U.S. involvement in NATO is necessary for the long-run stability of Europe, which now must absorb a unified Germany and several emerging democracies into its balance. NATO's secondary purpose has always been to keep Germany's power from frightening its neighbors. Europe needs a continued American presence to mediate continental politics.

For now, the one obvious concern that keeps the alliance together is the fear that the bear isn't dead; he's just hibernating. While Yeltsin was one of the architects of the Soviet Union's demise, the member nations aren't willing to gamble on his continued goodwill.

Yeltsin's war to crush secessionists in Chechnya hasn't eased western minds. And should Yeltsin fall, the potential of Russian nationalists--whether former communists or protofascists--to rise to power on Russia's economic woes remains a real danger.

As such, NATO has tentatively committed itself to expanding east, and this is a wise move. While the West fears harming the democratic movement in Russia by moving too fast with NATO expansion, it risks far more by moving too slowly.

If Russia reverts to its traditional expansionist policies, in the absence of NATO, a nationalist state will be tempted to extend its control into the power vacuum to the west. But if the U.S. extends its security guarantees to the Czechs, Slovaks, Poles and Hungarians, Russia will find Eastern Europe more than it can swallow.

On the other hand, if democratization in Russia is successful, the NATO alliance will eventually expand through the former republics of the Soviet Union and dissolve into a relatively benign, functionally impotent collective security framework.

Yeltsin is understandably concerned. While Clinton should make every effort to convince Yeltsin of the U.S.'s good faith, that doesn't change the fact that expansion potentially threatens Russian security, and it will indubitably lessen their influence in the region in the future.

However, the worst mistake Clinton could make would be to use the "Partnership for Peace" as a permanent substitute for NATO's expansion. Having two classes of security guarantees would likely make the partnership a vague and ineffective alliance. At the same time, the prospect of western expansion would still inflame Russian nationalists.

So NATO must expand to the east, choosing its allies from the most stable of the new democracies. While Yeltsin may disagree, the U.S. should assure him, "It's nothing personal."

Steven A. Engel's column appears on alternate Wednesdays'