As their ideological rivalry fades, the leaders of the two most important global powers sit down this week to a summit meeting in which their ability to shape world affairs is less certain than at any time since World War II.

This 18th meeting of U.S. and Soviet leaders since 1955 comes at a time when the Soviet-dominated Warsaw Pact has all but collapsed, the NATO alliance is struggling to find a new role as the Cold War fades from view, and the future of a united and powerful Germany, resistant to direction by outside powers, is the most urgent issue of international affairs.

Moreover, since President Bush and Soviet President Mikhail Gorbachev last met at storm-tossed Malta just six months ago, the internal stability and direction of the Soviet Union itself suddenly have become uncertain to a greater degree than at any time since the Bolshevik revolution. Like a seemingly solid corporation beset by a takeover bid, the future of the Soviet Union is now in play.

The problems of Bush are by no means as great as those facing Gorbachev. Yet at the beginning of the 1990s the United States finds that it must deal with increasingly powerful competitors among its economic, political and military allies.

The United States and the Soviet Union still possess about 55,000 nuclear warheads and bombs, about 98 percent of the world's nuclear weapons, but in other respects it is increasingly questionable to call them superpowers. The alliances that gave them clout far beyond their borders are sagging and the global reach of their political authority and national military power is shrinking. In trade and financial terms, increasingly the coin of the realm in international relations, the Soviet Union is a minor player with 4 percent of world trade and the United States a troubled economic power, buffeted by severe internal and external strains, accounting for 13 percent of world trade.

As the danger of all-out war or nuclear attack has sharply diminished, the world no longer holds its breath waiting for the news from the interaction of U.S. and Soviet leaders at the summit. "The two countries still have a lot of weight in the world," said a U.S. official who closely follows Soviet affairs. "At the same time, the relationship doesn't have the immediacy that it had some years ago, the sense that everything else in the world depends on this."

Georgi Arbatov, director of Moscow's Institute of the U.S.A. and Canada, said in a recent interview that the nature of the relationship has changed from concentration on "fears of each other, of how to somehow preclude a catastrophe," to creating a new structure for international life after the Cold War.

"The first dividend both countries had was to look around with clear eyes and see the world around us without distortions," said Arbatov. "We were too overwhelmed with our hostile relations {to do this} for quite a long time."

The Soviet Union's leading Americanist noted the decline in public fascination with the summits since Gorbachev came to power in 1985. "The first meeting at Geneva was dramatic, the second at Reykjavik was more dramatic. Beginning with the third {the 1987 Washington summit}, it lost in drama. I think it is very good. It should become an important but rather routine operation of meeting each year and having some eye-to-eye discussions and solving some of the problems. . . . It is on the way to becoming more routine and it is a normal way, the way to normalcy."

As the Warsaw Pact alliance of the Soviet Union fell apart and Germany moved toward unity, there has been a notable shift in U.S. diplomacy from concentration on U.S.-Soviet relations to a broader focus on U.S.-European relations, with the Soviets seen increasingly as just one of several important European powers.

Bush said in an interview with British Broadcasting Co. last week that among central questions for the summit are, "How does post-German unification Europe look? Who will be calling the shots? What is the role of the United States in terms of stability?" A presidential aide said such questions are not likely to be answered in one U.S.-Soviet summit, but will be pursued in "a series of summits over the course of the administration."

The day has passed when the United States and Soviet Union by themselves could decide much of anything about major issues in Europe. The questions Bush will discuss with Gorbachev this week will also be addressed at a proliferation of summits just ahead: the European Community summit June 25-26 in Dublin, at which the United States will not be present; the NATO alliance summit meeting July 5-6 in London; the seven-nation economic summit July 9-10 in Houston; a series of foreign ministers' meetings of the United States, Soviet Union, Britain, France and the two Germanys in June, July and September; and possibly a 35-nation all-European (CSCE) summit including the United States late this year.

In the era that is emerging, "the Soviets are not the key partner for us in many areas of global concern, such as economics and the environment," said Helmut Sonnenfeldt of Brookings Institution, who was involved in the preparations of the U.S.-Soviet summits of the 1950s and 1960s and who attended the five Nixon-Ford-era summits. The Soviet Union remains important in the military field, Sonnenfeldt added, but some of the most crucial military issues, such as security in the center of Europe, also involve many other players.

"In the past, the integrating factor in the West has been security issues but in the future the integrating factor will be trade, finance and economics," said Robert Hormats, a former White House and State Department official who is vice chairman of Goldman Sachs International. "In the non-military field there are three centers of power rather than two -- Japan and East Asia; the United States and Canada; Germany and Europe. The Soviets, not being an economic superpower, will play a diminished role in the world. The real concern about the Soviet Union today in economic terms is whether their economy will decline or collapse, with bad effects in Eastern Europe."

Prof. John Lewis Gaddis of Ohio University, a historian known for his writings on "The Long Peace" following World War II, said that "with the passage of time, what used to be a competitive relationship {between Washington and Moscow} has evolved into something like that of a couple of corporations who are competing in some areas and cooperating in others. The more that time goes on, the more the cooperative interests emerge."

What bothers Gaddis, though, is that, especially in Europe, the two global leaders are not playing the central roles they played from the 1950s to 1980s. "What has kept Europe stable and kept it free from war is that we were not on the sidelines but were actively involved," said Gaddis. He said it is particularly worrisome that, with the decline of the Warsaw Pact, there is no security structure in Eastern Europe that can engage the Soviets in the way NATO engages the United States and Western Europeans.

Outside of Europe, the United States and Soviet Union are still engaged in Third World struggles, but not nearly so deeply as during earlier decades.

Since Gorbachev came to Washington for his summit with President Ronald Reagan in December 1987, Soviet military forces announced and completed their withdrawal from Afghanistan, defying many predictions here that they would never pull out. That conflict continues at a lower level of intensity despite U.S.-Soviet diplomacy aimed at winding it up. Last week a National Security Council official, briefing reporters at the White House, declared that "it's hard to say at this point" that the Afghan struggle remains serious enough to exert a negative influence on relations between Washington and Moscow.

When Bush and Gorbachev met at Malta last December, the main bone of contention between them was Soviet aid to Nicaragua. Since then, the Sandinista regime was swept from power through the surprise result of the Feb. 25 Nicaraguan election, and the NSC official said last week that "we are somewhat satisfied with Soviet policy on Nicaragua." The official continued to be critical of Soviet aid to Cuba, which he estimated at $4 billion yearly.

The most urgent regional issue to be discussed at this week's summit, according to the White House, is the struggle between India and Pakistan over Kashmir, which the NSC official described as "deteriorating very rapidly and ominously." Here the limits of "superpower" influence are plain.

The Soviets earlier this month turned down the U.S. idea of a joint appeal to the two South Asian parties, perhaps because of the likely objections of India. Bush sent Deputy National Security Adviser Robert M. Gates to India and Pakistan to appeal for calm and the redeployment of military forces away from the battlefield. To date, however, the appeal from the U.S. president seems to have had no effect.