"It is an international duty, the duty of socialists and the duty of revolutionaries . . . to cooperate, however modestly, with other countries that are poorer than we."

"Africa is the weakest link in the chain of imperialism."

Fidel Castro, April, 1977

Internationalism has replaced domestic vigilance as the watchword of the Cuban revolution.

With its gorwing presence in Africa, Cuba is changing its self-image from that of beleaguered socialist David, surrounded by capitalist and rightist Goliaths, to that of a beneficient big brother who is ready, willing and able to affect events halfway across the world.

Judging by domestic support and stated government objectives, there are many signs that Cuban activity in Africa will increase.

That activity now stands at an estimated 10,000 troops in Angola, possible weapon and financial support for nationalists in southern Africa, and military advisors, technicians and physicians in Ethiopia and other African nations.

While there has so far been no public Cuban commitment to deploy troops beyond Angola, the option has undoubtedly been left open.

In speeches throughout Cuba over the past several months, marking events ranging from school graduations to national holidays, President Fidel Castrol has tallied the number of Cubans serving in Africa. He has promised his countrymen that the aid program, particularly its technical aspects, has just begun.

In a tone reminiscent of John F. Kennedy's inaugural promise to defend democracy both in the United States and abroad, Castrol has said that "anybody who is willing and ready to defend other people will always be ready and willing to defend his own people down to the last drop of his blood."

On a whirlwind African tour last spring, Castro visited Angloa. Mozambique. Libya, Tanzania, Ethiopia and Somalia. Souvenirs of the mission, from postcards to zebra hids to poster-size photographs of Castro with each of the African heads of state, are being exhibited nightly to a packed house at central Havana's "Club Pavillion."

The name of the exhibit is "We Are Internationalists," a slogan that trips as lightly off the Cuban tongue as the anti-Batista revolutionary cry Vencremost - "We shall overcome."

Both Castro's speeches and a daily barrage of articles in Granma, the Communist Party newspaper, supply Cubans with minutiae of current African battles and social problems that most can now recite, chapter and verse.

"In Ethiopia," said a Cuban film producer, "they have an average of one doctor for 80,000 people. In Cuba, there is now one per 1,000."

"That is why," he said proudly, "we can send doctors to Africa."

The troops in Angloa and more than 4,000 Cuban civilians scattered on missions throughout the world are portrayed here as proof not only that the Cuban revolution has trumphed at home, but that "we can afford the luxury of cooperating with others."

Far from the Latin American jungles, where Cuban revolutionaries fought alongside local guerrillas in the 1960s. Cuba has found a different kind of battleground in Africa. In Africa, Cuba is employing the Western blocs own tactics of calculated support of movements that have built-in possibilities of winning.

First, with Angolgan president Agostinho Neto's Marxist Popular Movement for the Liberation of Angola, and now in Ethiopia. Cuba has shored up existing socialist power structures with men, material and technical expertise.

While the battle in white-ruled southern Africa may be more of a guerrilla struggle, eventual victory is virtually assured.

Along with big-power "internationalism," Castro has also run into the problems of big power from other Africans, a nervousness among more moderate allies, and the necessity of making economics friends with distasteful partners.

Cuban military advisers were first reported in Ethiopia in May. But, while some Cubans hint that friends and relatives are fighting Eritrean rebels there, and the rebels themselves have accused Cuba of deploying as many as 5,000 troops, there has been no conclusive evidence that fighting troops have been sent beyond Angola.

Foreign observers here believe that Castro simply has not yet made a decision on whether to extend his military presence to other parts of Africa.

While Rhodesian black nationalist leader Joshua Nkomo said he came to Cuba last month to "look for arms," he left with only a vague hint that substantial aid had been offered.

Granma made a point of noting that following talks with Castro, Nkomo had said that "neither Cuban internationalist fighters nor fighters from other countries were participating in the war alongside the patriotic (Rhodesian liberation) forces, nor had they been requested to do so."

Particularly in Ethiopia, the prospect for armed Cuban intervention is more complicated than it was in Angola. The new Marxist government of Mengistubaile Mariam is fighting not only internal rebels but is also involved in a border war with neighboring Somalia, a country that has been supported by both Cuba and the Soviet Union.

On a June trip to Africa, Cuban Foreign Minister Isidoro Malmierca visited leaders in several of those countries, including powerful Nigeria. Nigeria's early 1976 support of Neto's Popular Movement was based in part on the understanding that the Cubans would leave as soon as the situation stabilized. Cuba is no explaining to them why the troops are still there.