As Americans and Europeans pause to honor the heroic Allied invasion of Europe 40 years ago, what can be said of today's world and its challenges? Diplomatic and other foreign policy experts cite some significant changes as well as achievements and problems:
* Despite concern about the danger of nuclear war, the world has enjoyed a long four decades of peace because of the strength of the Atlantic Alliance and nuclear deterrence.
* Both the United States and the Soviet Union have seen their status and influence decline with the emergence of a multipower, often fragmented world. The cold-war battleground has receded. From Africa to Asia to Europe, countries are acting more independently of the superpowers. The Soviets find themselves even more frustrated abroad than the US.
* Europe remains committed to collective security in the face of Soviet attempts to divide the alliance. But it is in a period of flux and generational change marked by undercurrents of unease about US ''unilateralism'' and differences of view about the Soviets and the nuclear balance.
* The fundamental problems persist: relations with the Russians, forging an international economic order that includes the developing nations, and the nuclear arms race. But because of changing relationships among countries, the problems have to be dealt with differently.
* Although the world grows increasingly interdependent, great numbers of Americans remain inward looking in their basic attitudes toward the world.
Disappearance of a bipolar world is not surprising, diplomatic experts say. American and Soviet power rose on the ashes of Europe and Japan. There was bound to be a correction from the immediate postwar period, in which the US dominated the world scene.
''The year 1945 was an aberration,'' a State Department officialsays. ''Historically there was always a balance between many countries. Now many other countries have come into being and they are evening things out. The lower classes are coming up and making things difficult for the parents.''
''Both superpowers have become less relevant today to world affairs than 20 years ago,'' comments William Hyland, editor of Foreign Affairs. ''There are areas where they kibitz but are not critically involved.''
Experts note, for instance, that Indonesia and other Southeast Asian countries are pursuing their own initiatives, that the US has little role on the Indian subcontinent, that the Iran-Iraq war has nothing to do with US-Soviet tensions, that southern African countries seem to be moving largely on their own , and that Europeans are preoccupied with sorting out their own problems in the Common Market.
And even while the US is offering aid in the Persian Gulf, the Arab oil states are making clear they do not want an imposing American military presence there.
''This trend has its healthy aspects,'' says David D. Newsom, director of Georgetown University's Institute for the Study of Diplomacy. ''It's something we have long wanted. But in places like the Middle East, where we do have important interests, the fact that we are less important to them is disturbing in the long term.''
''There's been a growth of regionalism and nationalism,'' Mr. Hyland says. ''Other countries are saying, 'We have our own problems. We have to take care of ourselves.' 'The Russians are coming' does not evoke response anymore.''
If this sometimes disconcerts US policymakers, especially in an administration that still tends to operate on the basis of a bipolar world, there is comfort in the fact that the Soviet Union similarly finds itself unable to control events. After a breakthrough in Angola and Ethiopia, for example, Soviet influence is receding. Many Cubans have left Ethiopia. In Angola there appears to be no great advantage to Moscow in the Cuban presence.
Then there is Afghanistan, where Moscow has yet to impose its will, and Poland, where only military rule keeps a population supine.
''People like the Soviets on their side only for expedient reasons,'' says Helmut Sonnenfeldt, guest scholar at the Brookings Institution, ''but no one feels close to them and the general impact of Soviet society on other countries and societies is much smaller than the US impact.''
Frustrated by their failures abroad - and Ronald Reagan's brand of anti-Sovietism - the Soviets have turned inward. But domestically, too, they are frustrated, experts say. The Soviet military understands that Moscow cannot maintain its international position without a strong economic base, but the leadership can only tinker with the system, lest it invite political challenge to its rule. Yet hunkering down is not all that easy.
''They have to allow East-West exchange to keep things stable,'' says a State Department analyst. ''The Helsinki process, detente, the European security conference have played a role in unleashing the internal dynamics of countries, and the natural forces in Europe have asserted themselves.''
In Western Europe, meanwhile, after 40 years of close transatlantic relations , many experts see a growing unease. Europeans as a whole have been put off by Soviet bullying and have accepted a bracing of NATO nuclear defenses. But there is chronic apprehension that the US is not prepared to use nuclear weapons on Europe's behalf and that Europeans must look more to their own resources.
This is compounded, say experts, by concern in some quarters about a US turning increasingly to the Pacific region, President Reagan's alleged hankering for confrontation, protectionist talk in Washington, and a US insensitivity to Europe over budget deficits and interest rates.
Adding to European uneasiness is a discontent with its own economic performance, and its inability to create jobs and keep up with the high-tech revolution in the world. Contrasted with this is the capacity of the Americans once again to pull themselves together and demonstrate their economy dynamism and innovation.
''There has been a slight shift in European attitudes,'' says Robert Bowie, a former senior State Department and Central Intelligence Agency official. ''The Soviet Union is seen as less significant, though not written off. The US is viewed as having emerged again - not as a necessarily attractive society - but as powerful economically, militarily, politically, and capable of acting on the world scene but doing so without a sense of mutuality.''
The Atlantic Alliance is not put in question. There remains strong European public support for it and a broad defense consensus. But there is a sense of European groping for how to adapt it to changing conditions.
''Another 10 years and it will be 50 years since the war, so change is indicated,'' says Mr. Sonnenfeldt. ''And if that indicates a greater European coalescence, that would be desirable. But there is uncertainty now. The French are concerned about where the Germans are headed. There is concern about where the Russians are headed in Eastern Europe. We don't know what's ahead.''
Americans, for their part, are troubled by nationalistic tendencies in Europe and the squabbling within the European Community, exemplified by the split on agricultural policy between Britain and the Continent. Persisting in the US, too, is the feeling that Europeans are not pulling their share of the defense burden.
''The Europeans are so skittish they can't come up with a rational drawdown of the American presence there,'' says R. Gerald Livingston, acting director of the American Institute for Contemporary German Studies. ''There has to be more willingness to countenance the fact that we have obligations elsewhere.''
''The US has an ambivalent feeling,'' says former Defense Secretary Harold Brown. ''It wants Europe to be more cohesive, but it does not want it to go off on a different tack. Europeans will take somewhat more responsibility for their own defense but not enough to allow the US to make major withdrawals.''
One of the challenges ahead, says Dr. Brown, will be the inevitable social, historical, and generational change in Europe. ''The generation that remembers the war is passing from the scene,'' he says, ''and the people who will come to power in Europe and the US in the next 10 years will have the advantage of having no hang-ups but the disadvantages of not having experienced the war. The new generation is challenging the alliance, but even the older generation tends to get tired. So the alliance will be increasingly challenged.''
Assessing today's climate, observers also point to a persisting national self-centeredness among Americans, who remain averse to involvement abroad.
''We have been immersed in our own economic problems, and third-world issues do not stir interest,'' remarks Theodore L. Eliot Jr., dean of the Fletcher School of International Law and Diplomacy. ''Americans do like to hear that 'we're standing tall in the world.' But even though our economic involvement in the world is increasing, this does not translate into a will to prepare our young people for careers in which international factors will be playing a bigger role.''