LONDON, JULY 4 -- After four decades as the West's most powerful bulwark against communism, the 16-nation North Atlantic Treaty Organization will be trying this week to adjust to life after the revolution it helped spawn in Communist Eastern Europe. To many Europeans, NATO suddenly seems like a pact without a purpose, an army without an enemy, struggling to preserve its central role and influence in a dramatically different Europe emerging from the ashes of the Cold War.

Call it a victim of its own success or a dinosaur that has outlived its usefulness. Either way, from the viewpoint of many of the Europeans it was designed to defend, NATO must find a new reason to exist. If it fails, the United States, the alliance's main guarantor, may face a parallel loss of influence in Europe.

Many believe NATO's Cold War doctrine is rapidly becoming obsolete, its military muscle on the verge of swift contraction, its leadership divided and uncertain about how to prepare for a world in which the major conflicts are likely to occur not between East and West, but among resurgent nationalisms within the former East Bloc and in the Third World.

NATO's search has infused a sense of urgency and unease into the meeting of its leaders here Thursday and Friday. George Walden, a member of Britain's Parliament, likens it to a swan swimming upstream: "Serenity on the surface, but paddling like hell underneath."

The meeting's purpose, said a senior aide to British Prime Minister Margaret Thatcher, is "to strike a balance between preserving the fundamentals of NATO {while} adapting to the new circumstances which we see all around us."

Yet because these leaders seem eager to avoid open conflict among themselves at a time of uncertainty and to present a reassuring appearance to Moscow, the most crucial issues about NATO's future are unlikely to be decided or even openly discussed.

"There has to be an evolution of NATO," said NATO Secretary General Lord Carrington in recent testimony before a House of Commons committee. "You have to call it NATO. You have to make the Americans feel that it is still NATO. But it has to evolve into something slightly different.

"The structures of NATO are flexible enough to accommodate almost anything. Whether the countries of NATO are flexible enough to adjust themselves to what is happening is another matter."

After nine months of the most sweeping changes since the close of World War II, the view from Europe is both visionary and wary, ecstatic yet fearful. Twice in this century, Europe witnessed the birth of hopeful post-war eras, only to see them suffocated by a new confrontation. It remains the world's most militarized continent.

For that reason, each of NATO's members -- and even many of the East European countries that until recently were classified as its foes -- wants the alliance to continue in some form. Still, few see the new Soviet Union as a military threat, although it still fields Europe's largest army and the world's most extensive nuclear arsenal. These leaders express greater fear of Soviet economic collapse and national disintegration and the ethnic confrontation such a collapse could bring about.

All are anxious to preserve some role for the United States as a force for stability and a symbol of Western values in the brave new Europe that is rapidly emerging. Britain and France fear some of their own power and prestige may fade as NATO recedes and their independent nuclear forces shrink almost to their vanishing points under future arms-control treaties.

Spurred by popular pressure at home, every member state except France and Turkey is engaged either in a review or a reduction of defense expenditures. The United States and Britain, the two countries most reluctant to change strategies, have proposed some of the biggest cuts.

At the same time, Europe is reinforcing institutions that may sap NATO's influence -- the 12-nation European Community and the Conference on Security and Cooperation in Europe. In so doing, the Europeans may diminish American influence on the continent -- especially with a more powerful Germany -- since Washington has no role in the EC and a diluted role in the 35-nation CSCE.

Most Western commentators suggest that the EC, growing almost daily in economic and political power, will be the centerpiece of the new order and the vehicle for integrating the nations of Eastern Europe into the West. They also say the new Germany will be more firmly anchored by a robust EC than by the more ambiguous bonds of NATO. Even some of NATO's most enthusiastic supporters, including Lord Carrington, have called for the EC to begin taking on a new security role.

Moscow and many of its former Warsaw Pact allies are pressing for new security arrangements under the umbrella of the CSCE. Despite its commitment to NATO, West Germany has also spoken of the CSCE as a key element in the new order -- one that analysts say inevitably would impinge on the alliance.

The United States and its partners have put forward tentative ideas for transforming NATO from Cold War eagle into new-era dove. This week's summit is likely to endorse the concept of nonaggression accords between individual Western states and members of the collapsing Warsaw Pact, and it may approve reorganization of NATO ground forces in West Germany into multinational units. It also will pay at least lip service to a larger security role for CSCE -- but as a complement to, not a substitute for, NATO.

One of the most widely discussed new ideas is that NATO could play an enhanced "out of area" role in such places as the Middle East. But critics say such proposals run up against the hard reality that when it comes to non-European issues like the Arab-Israeli conflict, the United States and its allies seldom see eye to eye.

In some ways, many Europeans contend, the United States is dealing itself out of Europe's future. Along with reductions of troops and weapons has come a decline in American economic muscle. There are no Marshall Plans for the former Soviet Bloc. Indeed, President Bush encouraged the European Community to play the leading role in revitalizing Eastern Europe -- and to foot most of the bill.

"NATO may still provide the main vehicle for American influence in Europe," said Lawrence Freedman, defense analyst with the Department of War Studies at King's College here. "But it will not provide the same opportunities for American leadership."

West Germany holds the key to NATO's future, not only because it is the most influential nation in Europe and the largest after the Soviet Union, but also because most of the alliance's troops and weapons are stationed there. But despite repeated assurances from Chancellor Helmut Kohl about his commitment to NATO, its future in the new Germany is a hostage to fortune -- specifically to the complex bargaining between the West Germans and Soviets over German unification.

Kohl and Foreign Minister Hans-Dietrich Genscher express eagerness for an understanding with Moscow in time for an all-German election on Dec. 2. They call for the new Germany to be part of NATO and are prepared to allow 360,000 Soviet troops to remain in the old East Germany for up to a five-year transition period and even to pay part of their salaries and airfare home. They have pledged $3 billion in guarantees for bank loans to Moscow and have championed the case for a Western commitment of up to $20 billion in aid to bail out Soviet President Mikhail Gorbachev.

When it comes to NATO, the West Germans have already proposed a series of radical alterations in defense doctrine and a shrinkage of forces designed to render the alliance less offensive to Moscow. They appear prepared to go even further to win Gorbachev's acquiescence -- and to preserve their electoral lead with voters dubious about the need for nuclear weapons and large foreign forces on German soil.

"The unification process is driving everything," said Josef Joffee, foreign editor of the newspaper Sueddeutsche Zeitung. "Right now it's a bazaar -- the Russians aren't saying what their final price is for unification, they're simply asking what the Germans are willing to pay. Their position looks weak on paper. But in my view they can win a lot more concessions from Kohl and Genscher if they play it tough, because the Germans want unification so badly."

An eclipse of NATO could mean further marginalization of Britain, which for years has used the alliance as a means of maintaining its place in the sun. Nonetheless, domestic politics has forced Margaret Thatcher's government to make some deep cuts in defense spending and pledge even deeper ones, despite objections from the chiefs of the various military services and the country's extensive arms industry. More than a mere defense review, the new appraisal is "an identity crisis," said Parliament member Walden. "It's a way of asking who we are and where we think we are going. If we're not for NATO and the Cold War and holding things together, then what are we for?"

Those are questions France so far has opted to avoid, at least in public. German unification is a prospect that French officials must endorse publicly even if they dread it, and they are clinging to the status quo. They say the defense budget for next year will show no decline in spending and that no serious plan has been drawn up for troop cuts.

In the long term, President Francois Mitterrand has spoken of a "European pillar" within the Atlantic alliance that presumably would assume some of the functions now undertaken in concert with the United States. But a senior aide said this week that the issue would remain moot for the near future. "There are too many things in motion right now, including German unification and the changes in the Soviet Union, to enable us to give a clear definition to this term now," he said.

Correspondent Jim Hoagland contributed to this article from Paris.