With the winds of Soviet President Mikhail Gorbachev's change sweeping the continent, the old truths that have defined NATO's purpose are fast disappearing.

The changes are more than mere cosmetic "Gorbacharm." Moscow is sharply cutting its forces in Europe, prying open its political process to broader participation and aggressively seeking new ties to Western Europe.

Nor is Eastern Europe the monolithic, Kremlin-ruled empire it once seemed. Instead, the "satellite" states are charting their own courses that increasingly diverge from Marxist doctrine, and are seeking their own accommodation with the West.

These changes have caught NATO off guard and left it unsure of its own purpose. Having struggled so long and so hard against the communist military threat, the alliance now faces an uncomfortable new question: Has it become irrelevant?

"We have won the Cold War - everything NATO has stood for is happening," said a senior U.S. official. "The question is, what do we do for an encore?"

At the center of that question is another with profound implications for America's future security and economic vitality: With Europe poised to become an economic bloc rivaling the United States, is America about to lose its accustomed place as leader of the West?

Such questions have immense importance not only to geo-strategists, but to millions of ordinary Americans who twice this century sent sons and husbands to die on European battlefields and then paid to rebuild the war-ravaged continent.

At stake, too, is America's critical access to West European credit and markets, and its role in shaping Eastern Europe's emergence as a group of independent democratic nations that increase world stability rather than provide tinder for a new global conflagration.

Sen. Al Gore, D-Tenn., a member of the Senate Armed Services Committee, says the new realities "call for the withering away of alliances . . . but we have no clear model as yet of how Europe might be organized without them."

With Gorbachev pushing his own agenda for change, time for NATO and the United States may be running out.

"It is fashionable these days to say the Cold War is over and the West has won," Robert Legvold, a Soviet expert at Columbia University, recently wrote in Foreign Affairs magazine. "What may people do not realize is that we in the West are in danger of ending it on Soviet terms."

A senior U.S. official said the Bush administration is studying how NATO might restructure itself, but has "no clear answers" and probably will be unable to offer a blueprint at a NATO summit in Brussels in May.

Meanwhile, the Europeans appear to be providing their own answers - ones that don't necessarily include U.S. leadership.

Western Europe is well along toward restructuring itself into a unified economic bloc. In 1992 it will dismantle most domestic regulations restricting the flow of trade and finance - creating what analysts believe will be an aggressive economic power capable of competing equally with the United States.

A similar movement away from the United States is taking place on the military front. Germany and

France are creating a joint infantry brigade and are planning an all-European division, while Great Britain and France are discussing joint nuclear weapons coordination.

"It is inevitable that there will be a reduction of American forces in Europe, which will mean we lose influence on the continent," said Robert Hunter, director of European studies at the Center for Strategic and International Studies and a former White House National Security Council officer.

In the future, Hunter said, "It will be interesting to see if we can play the game there at all."

All this is a dramatic change from 40 years ago, when there was little question about the Soviet threat or America's role in meeting it. Many believed that all that stood between the 2.9 million-man Red Army and the smoking ruins of Europe was a small U.S. garrison in West Berlin.

Typical was a secret British assessment of 1948 warning that the Kremlin "is actively preparing to extend its hold over the remaining portion of continental Europe and subsequently over the Middle East and no doubt the bulk of the Far East as well ... leading to the establishment of a World Dictatorship."

Nevertheless, the North Atlantic Treaty signed by the United States and 11 European countries on April 4, 1949, was primarily a pledge of political cooperation and mutual defense.

Not until a year later - amid fear of a new world war prompted by the communist invasion of South Korea - did the United States put muscle behind the treaty by dispatching four Army divisions to Europe and naming Gen. Dwight Eisenhower as head of Allied forces.

It was intended, President Truman told Congress, as an "interim" measure. Eisenhower noted at the time that if U.S. troops were still in Europe a decade later, "then this whole project (NATO) will have failed."

Four decades later, that original reinforcement of 94,000 American troops has grown to a permanent force of about 350,000, equipped with chemical and nuclear weapons. Allied strength there has been more than matched by the Soviets.

The "great antagonism" that these forces symbolize, Ohio University historian John Lewis Gaddis wrote recently, "has become encrusted, over the years, with successive layers of routine, custom, tradition, myth and legend."

Indeed, there is increasing unhappiness about NATO on both sides of the Atlantic.

The United States gave Western Europe the equivalent of $185 billion for postwar reconstruction, and now pays about $160 billion annually in defense of Europe - more than the rest of the allies combined.

But the ground on which these commitments are based has shifted significantly. Western Europe has recovered from the war's devastation to become an economic powerhouse.

That shift has prompted a rising American impatience with the cost of securing an economic competitor.

As a recent House Armed Services Committee report stated, "Many Americans feel that the United States has incurred all the burdens of empire and few, if any, of the benefits."

Similar irritations rankle NATO members on the other side of the Atlantic, where there is growing opposition to the low-flying aircraft missions and tank maneuvers required to keep U.S. and NATO forces combat-ready.

As Europeans become more assertive of their own clout, it is becoming increasingly difficult for the United States to maintain its military bases there. Spain already has ordered the removal of an F-16 fighter wing, and tough negotiations lie ahead in Greece and Portugal.

Despite the lure of eventual reunification with their countrymen across the Iron Curtain, West German officials insist they are not "drifting away" from NATO.

Other Germans disagree. West German security analyst Harald Ruddenklau says West Germany's security interests already have begun to split away from the alliance, particularly on the issue of European-based nuclear weapons.

Once the Germans' support for NATO disintegrates, he said, "Western powers will be unable to defend effectively on German soil their strategic interests regarding the Soviet Union."

Such risks, though, present the United States and NATO with new opportunities to restructure their military component and forge improved political and economic relations.

"This is one time that an anniversary might actually mean something," said Hunter. "The object here is not to have alliances, but to have free and independent, and secure, societies. For the first time in years we can begin to tackle these issues honestly."