KENNEBUNKPORT, Me., Aug. 31— Just when the United States had adjusted to the idea that the Kremlin had turned from implacable enemy to benign adversary, the breathtaking events in the Soviet Union have forced the country to confront the possibility that soon there may be no adversary there at all.

The sudden prospect that the Soviet Union will cease to exist in its current form has implications that go far beyond Soviet-American relations, and the Soviet upheaval has already forced the Bush Administration to begin re-examining its fundamental policy goals.

So far, Bush Administration officials, as stunned as everyone by the events in Moscow, are focusing on short-term problems and making only modest changes in policy. Direct Aid to Republics

The White House made clear this week, for example, that it was prepared to send humanitarian aid directly to the republics, bypassing the collapsing central Government in Moscow. It also was encouraging the Soviet Union to maintain at least an economic union, and taking advantage of the upheaval to press Moscow to reduce military spending.

For 45 years, the Soviet Union has been the lens through which American foreign policy around the world and American military thinking, especially nuclear strategy, have been focused.

The threat of Communism has been at the heart of virtually every crisis the United States has faced since World War II, crystallizing national resolve in such seminal events as the Berlin airlift of 1948 and the Cuban missile crisis of 1962, or convulsing American society in darker passages, like the Vietnam War. Pervaded American Life

Fear of Communism and the Kremlin has touched almost every aspect of American life — literature, theater, the movies, television, the universities, main street.

Always painted in menacing shadows, the Kremlin has been a powerful and at times distorting influence on domestic politics since the Bolshevik Revolution in 1917, from the anti-Communist raids led by Attorney General A. Mitchell Palmer in 1919 to the witch hunts and blacklists of the McCarthy period in the 1950’s to the Reagan Administration’s politically divisive efforts to roll back Soviet influence in Nicaragua in the 1980’s.

The effort to repel Communism was the binding force of the modern conservative coalition, helping to keep Republicans in the White House, and the specter of the Soviet enemy has helped define the nation’s image of itself as the defender of threatened freedoms. The Soviet Union was the yardstick by which Americans measured their successes and their place in the world: the fear of losing the space race and falling behind in science when Sputnik soared into orbit in 1957, for example, or the national jubilation when sledgehammers tore into the Berlin Wall in 1989, and now for many a sense that the American-led stand against Communism and the Soviet Union has succeeded beyond anyone’s expectations. The ‘New World Order’ Ages

When President Bush offered his “new world order” a year ago to explain why the nation had to face war with Iraq during another tumultuous August, he grounded his vision on the assumption that there would continue to be a Soviet Union — much like the one that had existed since 1917, just friendlier and less of a military threat.

Last week, the President seemed to acknowledge the passing of that epoch. “Out of this change in the Soviet Union, if we handle it properly and if things keep going forward instead of slipping back, there’s an opportunity for a vastly restructured national security posture,” he said.

Cautious as ever, he added, “It’s way too early — way too early — to get into that.”

But even as he spoke, his Government was in the early stages of what eventually may be a fundamental rewriting of American policy, and a new debate was quickly developing in Washington about whether to make deeper cuts in the Pentagon budget.

The short-term questions the Administration is studying are perplexing enough: Who are the new people taking over the Soviet Union? Will Mikhail S. Gorbachev or Boris N. Yeltsin emerge the nation’s leader? Who is in control of the Soviet nuclear arsenal? How much food will the Soviet Union need this winter?

“All we can do now is to try to do everything we can think of to position ourselves to be able to move rapidly when the situation clarifies,” a senior official said. More Fundamental Questions

But beyond these questions are more fundamental ones, each of which may require sweeping changes in American policy. Will the central Soviet Government survive, and if not, what will emerge? How will the United States do business with four or five or more Governments where there is now one? What are the responsibilities of the United States as the only superpower in what used to be a two-superpower world? Against whom is the United States now arming itself militarily and ideologically?

“We’ve got to begin to lay an intellectual base for U.S. involvement in the world, a rationale that people understand and support and around which you can build a consensus, as was done with containment,” said Representative Lee Hamilton, Democrat of Indiana. Mr. Hamilton was referring to the goal of “containing” the Soviet bloc, which was set soon after World War II and has been the driving force behind American policy on the Soviet Union since then.

Already, the upheaval in Moscow has complicated a central aim of American foreign policy over the last two years: maintaining influence in Western Europe. Justification for NATO

“We’ve been talking for some time now about how to manage the breakdown of the cold war institutions, like NATO, without boxing us out of Europe,” an Administration official said. “That was hard enough with the collapse of the Warsaw Pact and it’s much harder now.”

The United States will be “looking to the European Community to pick up a major share of the financial burden” as Soviet republics emerge as countries, an American official said.

The Bush Administration was confident before the coup that it had forestalled the demise of the North Atlantic Treaty Organization, but that debate is now rejoined. What will justify NATO’s existence if there is no Soviet military from which to protect Western Europe?

Caspar W. Weinberger, the former Secretary of Defense, argued that the military threat was not gone as long as whatever form of country emerged retained an enormous nuclear arsenal and a large conventional military.

But Senator Richard Lugar, Republican of Indiana, said the United States should consider a security system that might include Poland, Hungary and even some former Soviet republics, like the Baltics, after they are free. “We ought to take a careful look at people who share our same ethos for democracy and market economics and right now are in a netherworld with regard to security,” Mr. Lugar said. Questions About Asia

The Bush Administration also faces important questions about its dealings with Asia, where the Soviet Union is less important and Japan is increasingly powerful.

Domestically, the Administration will argue that there is a need for a strong American military, citing the threat posed by Iraq last summer as a clear example. That may buy some time. But it is increasingly apparent that with the potential breakup of the Soviet Union, the current basis for American nuclear policy may not be enough to hold a consensus for military spending.

Despite all the changes of the last two years, that policy is still based on the principle of “strategic deterrence” — the amassing of nuclear weapons, initially to deter a Soviet attack on Western Europe and more recently to deter a Soviet nuclear attack.

The American intelligence community, as well, is being challenged to change from an organization devoted to spying on the Soviet Union and thwarting its foreign policy to one that deals with a more diverse world.

As perceptions and attitudes change, longstanding principles are already colliding with pragmatic goals. In the Soviet Union, the United States is torn between its support for self-determination and the Administration’s belief that central control over the military, trade and foreign aid and some aspects of foreign policy is desirable to maintain stability. Political Considerations

Politically, Republicans have held on to the White House in large measure because of the perception that their party has been better equipped to safeguard the national security, even if opinion polls say that they do not handle the economy as well as Democrats.

But the disappearance of a Soviet enemy stands that notion on its head. It may no longer be enough simply to argue, as Mr. Bush does, that the United States has a responsibility to keep the peace and, in fact, is the only country that can do so.

“People are very proud of the fact that the United States is the superpower and the leader in the world,” Mr. Hamilton said. “At the same time, they think we can better solve our problems at home if we scale down our efforts overseas. They’ve got a real doubt in their minds about whether they want to pay the taxes and send the young men and women abroad in support of that role.”

As the Soviet Union undergoes its metamorphosis, even vocabularies will haveto change. American officials now grope for a word to describe the Soviet Union. Certainly, the word “Kremlin” is losing its symbolism as the monolith of Soviet power. Hard-line, conservative, liberal, Communist, all have new meanings — or no meaning at all.

There is little in the modern American experience to prepare Washington for this upheaval.

“The challenge now is the transformation of a vast empire and that was never part of our agenda,” said Richard Perle, former Assistant Secretary of Defense.

Mr. Perle added: “Our international agenda for 40 years was much more modest, to cope with the Soviet Union, which we saw as a military threat, not to transform the Soviet Union itself. Even the rollback policy, which was considered adventurist in the cold war, never contemplated involving the United States in the transformation of Soviet society or the Soviet republics.”

Photo: President Bush returning to the club house after a downpour cancelled his golf game yesterday in Kennebunkport, Me. (Associated Press)