NATO's top leaders will meet here Tuesday for a two-day summit that some observers consider a major event in the evolution of U.S.-West European relations and others call a gathering for a group portrait to show the voters at home.

In actuality, as interviews with U.S. and European diplomatic sources indicate, the truth seems to lie somewhere in between these assessments.

The summit conference, which comes in the 29th year of the 15-nation North Atlantic Treaty Organization alliance, does have a lot of serious business on its agenda. In one way or another, it involves the central question of how NATO, in the years just ahead, can best fulfill its role as the principal western counter to Soviet military power in Europe.

As is inevitable on the periodic occasions when a NATO meeting turns into a heads-of-government summit, the political aspects of that question tend to get more attention than such overtly military considerations as strategy, weapons and budgets.

When President Carter gets together with his fellow governmental chiefs (only French President Valery Giscard d'Estaing will not attend), their main emphasis will be on solving, or at least patching up, the internal strains endemic in an alliance whose members have so many divergent interests.

Chief among them this year will be the complicated equation posed by Greek-Turkish tensions over Cyprus and the threat the tensions pose to NATO's southeastern flank defenses in the Mediterranean.

Also high on the agenda, many sources predict, will be discussion of a problem that lies outside NATO's normal sphere of interest but that is of concern to most of its members: the increase of Soviet-Cuban military involvement in Africa as a means of extending communist influence on that continent.

And, there are several other matters having a strong bearing on western interests and the future effectiveness of the alliance. Still, without minimizing their seriousness, most of the sources interviewed agree that the political cohesiveness of NATO - looked at across-the-board - is in relatively good shape.

That's in marked contrast to the situation a few months ago when high government officials on both sides of the Atlantic worried openly about the danger of NATO falling victim to drift and malaise.

Although there were a lot of reasons for that situations, most were rooted in the newness of the Carter administration and the difficulty European leaders had in taking the measure of Washington's new team.

For many European governments, the experience was especially traumatic because they found themselves trying to figure out a president who nurtured an image as a Washington outsider, who surrounded himself with unfamiliar faces and who preached exotic notions such as human rights.

That led to misunderstandings on a number of issues - that Washington might downgrade its interest in Europe in favor of a romantic emphasis on the Third World, that its human rights policy would unduly antagonize the Soviet Union and endanger detente, that its pursuit of a strategic arms limitation talks (SALT) agreement with Moscow would adversely affect the balance of Western Europe's defenses.

While it would be misleading to assume that these issues have been resolved, diplomatic sources generally agree that most of the sharp edges at least have been blunted.

In the interim, the Europeans and the Carter people have come to know each other better; new working relationships have been forged, and, while there are still a lot of questions in Europe about Carter's ability and the wisdom of his policies, there does seem to be a better understanding of what his administration is trying to do.

In this respect, many sources point to the attitude of West Germany, which economically, militarily and geographically is NATO's most important European member. Under the leadership of Chancellor Helmut Schmidt, it also has been the most openly critical of Carter.

The West Germans still have a lot of complaints about the United States. But most of them involve economic and monetary questions that will be thrashed out in other forums such as the economic summit conference scheduled for Bonn in July.

Where NATO is concerned, Bonn-Washington relations appear to have improved considerably. The controversy over U.S. production of and West German use of neutron arms has been shoved to the back burner; Washington has eased Schmidt's fears about the United States dealing over the heads of its allies in the SALT negotiations, and German sources say they anticipate "no fireworks" when Schmidt and Carter get together this week.

That reinforces the general assumption that the summit will take place in an atmosphere of good feeling, that Carter will have a good opportunity to try to impress himself and his ideas more strongly on his European counterparts, and that there will be a lot of potentially fruitful give and take on how to solve the alliance's most pressing problems.

In regard to specifics, there will be an obvious effort to use the summit as a wedge to help the administration in its uphill fight to convince Congress that it should lift the U.S. embargo on arms to Turkey.

This message is expected to emerge in a variety of ways: in what's communicated to Congress about Carter's talks with Turkish Prime Minister Bulent Ecevit and Greek Prime Minister Constantine Caramanlis and in the expressions of concern that other NATO leaders will make about the danger of the embargo causing Turkey to play an increasingly limited role in NATO.

Of potentially even greater interest will be what the summit says and does about means of countering the Soviet-Cuban presence in Africa. That subject has been at the top of the administration's foreign policy priority list in recent days, and the Europeans have been advised that it's something that Carter urgently wants to discuss.

That effort began Friday night when France's Giscard met with Carter at the White House. A joint statement released after Giscard returned to Paris said the two leaders "agreed that concerted action with the African countries is necessary to promote security and development in that continent."

Still unclear is what form such concerted action might take. Various sources say that ideas for possible discussion at the summit range from merely increasing economic assistance from the NATO countries to Africa through attempting to promote an African peacekeeping force under the Organization of African Unity to trying to find a country will act as a western counter to the Cuban military forces on the continent.

In respect to this last idea, the sources say, there has been vague but persistent talk about trying to induce Morocco, a former French colony that still has close ties to Paris, to use its forces as a surrogate for the West in Agrican regional conflicts, much as Cuba has acted for the Soviet Union.