It was only one sentence in President Carter's Naval Academy speech yesterday - an assertion that "many countries are becoming concerned that the nonaligned movement is being subverted by Cuba . . ."

But, with those words, the president revealed a key element in his administration's evolving strategy for countering Soviet involvement in Africa and other parts of the Third World.

Specifically, the strategy calls for trying to isolate Cuba from the nonaligned bloc of nations by protraying it as a totally dependent Soviet satellite acting as Moscow's military surrogate in Africa.

The objective is to put President Fridel Castro's regime in a position where it will be forced to choose between losing credibility and influence among the nonaligned nations or pulling back from foreign military adventures such as its commitment of 40,000 troops and technicians to Africa.

Underlying this strategy, administration sources say, is Washington's belief that Cuba is pursuing two contradictory goals: it is bidding for a leadership role within the nonaligned bloc, while aiding a superpower to interfere in Third World conflicts.

Such inconsistencies are not uncommon in the loosely defined arena of nonaligned politics. As one administration source admits:

"There are lots of ostensibly nonaligned countries that actually are three-quarters in the pocket of one of the superpowers. The Soviets have Cuba, and the United States has its own group of barely disguised client states, most of them Latin American, within the nonaligned bloc."

The difference," he adds, "is that Cuba is compaigning hard and seriously to get in the bloc's leadership inner circle at a time when its activities in Africa make its position in the Soviet pocket more glaring and exposed."

In the past, administration strategists note, the leaders of the nonaligned bloc - such countries and Yugoslavia, India, Indonesia and Sri Lanka - have sought to avoid close identification with either superpower.

The administration's new line of attack was first foreshadowed by Carter in his May 25 Chicago press conference when he accused Cuba of helping to train and equip the rebels who invaded Zaire last month.

"It is a joke to call Cuba nonaligned," the president said in Chicago. Since then, administration officals have continued to pound at that theme, with Carter charging yesterday that Cube "is obviously closely aligned with and dependent upon the Soviet Union for economic sustenance and for political and military guidnace and direction."

Whether this approach will have the desired effect among nonaligned leaders is a matter of some debate among administration officials.

Despite his unquestioned dependence on Soviet economic aid, Castro always has taken a highly independent line in his foreign policy. His government, which won power through guerrilla warfare, has always asserted that it has an oblilgation to support "wars of liberation" throughout the Third World.

During the 1960s, for example, that attitude brought Castro into frequent conflict with the then prevailing Soviet position that communist movements in most Third World areas should seek to achieve power by working within the established system rather than through guerrilla warefare.

Castro set up the Havana-based Latin American Solidarity Organization, which gave active support to Latin guerrilla movements and other groups working in opposition to the orthodox, Moscow-line communist parties of the region.

Although its principal focus was on Latin America, this organization, known as OLAS after its Spanish initials, established close ties with like-minded governments and groups throughout the Third World.

In recent years, Castro has been more subdued in his support of armed revolutions - largely because of economic difficulties that made him more dependent on Soviet aid to keep the Cuban economy on keel. At present, Moscow is putting more than $2 billion a year into Cuba through purchases of Cuban sugar and subsidized supplying of Cuba's oil requirements.

This financial backing undoubtedly is a powerful factor in Cuba's decision to act as Moscow's partner on the scene in Africa. But anyone who has dealt with Castro is quite aware of his old attitudes about exporting revolution and knows that he probably would have become a willing confederate of the Soviets in Africa even without economic inducements.

Among the people who know this are many of the nonaligned leaders that Washington hopes to influence - such Africans as Zambia's Kenneth Kaunda and Tanzania's Julius Nyerre and such Caribbean spokesmen as Jamaica's Michael Manley and Guyana's Forbes Burnham.

The unanswered question, administration sources concede, is which argument these leaders will believe about Castro's role in Africa - the U.S. charge that it's a sign of Soviet domination or the Cuban response that it's an anti-imperialist action totally consistent with the principles of nonalignment.