Far from heeding President Carter's warnings against continued Cuban military activity in Africa, Fidel Castro is quickly enlarging his expeditionary force in Ethiopia and keeping his huge Angolan force at its present level of roughly 20,000 troops and a few thousand civilians.

Thus, this assessment of the first hard wave of Carter's anti-Castro rhetoric following his earlier sweet talk about reestablishing full diplomatic relations with Cuba: zero impact. That raises an ominous question for the second year of the Carter presidency. Does the United States lack the means to deal with Castro's aggressive foreign policy, now based on the export of military power?

So far, the answer is yes, although top U.S. officials are seeking a new policy. Clearly, it has been revealed to the President's top foreign-policy men that a warning from the United States no longer brings results.

"Frankly, I must say that our recent efforts to influence Prime Minister Castro have done nothing so far," one top presidential adviser told us. Accidentally or not, the Carter administration's reluctant decision to publicize rising Cuban and Soviet military involvement in Angola and Ethiopia has coincided with an increase in Cuban forces in Ethiopia.

Cuban pilots may now be flying combat missions for Ethiopia's revolutionary government. Cuban military advisers, originally intended only for training purposes in Ethiopia, are now taking part in combat missions. Cuban troops are essential to keep in power the Marxist regime of Agostinho Neto in Angola.

Most significantly, Castro's propaganda organs at home are for the first time boasting about the military blessings the little island nation of Cuba has brought underdeveloped Africa. The cover of the magazine Siempre shows a massive, Soviet-built Cuban tank in the jungle, with an African on top armed with a bow and arrow.

While Castro slowly escalated his African adventures, the extent of the commitment was concealed back home. Cuban expeditionary troops were recruited from widely spaced villages and towns, apparently to diminish the negative effect of growing casualties. That toll is estimated at between 3,000 and 4,000 killed and wounded - equal, in terms of the U.S. population, to nearly 100,000.

Now, the time for possible concealment seems long since past. Ambitious to become leader of the revolutionary world, Castro is trying to make a domestic virtue of his foreign entanglements.

In attempting to quickly move the United States from spectator to actor in the escalating African drama, the Carter administration is more worried than it lets on about Cuba's rising involvement in Ethiopia. Some highly placed experts glumly predict that Castro may be aiming at a Cuban force in Ethiopia on the same scale as the 20,000-man force far to the south in Angola.

In both Angola and Ethiopia, Soviet arms and equipment supply the war material in ever-increasing amounts. An estimated $800-million worth has been sent to Ethiopia alone since the Soviets switched clients from Somalia to Ethiopia.

But Carter administration officials no longer cheer the prospect of a Fidel Castro "bogged down" in Angola or Ethiopia. Rather long-term, escalating Cuban influence from one end of the continent to the other is beginning to be viewed here with more realism. The Cubans signal to the rest of the world that the Soviet Union, exploiting Cuban fighting men using Soviet equipment, has freedom of action almost anywhere it wants in turbulent Africa; the United States, claiming to be above the battle, appeals to sweet reason and issues soft complaints.

The mood in Carter's administration at last seems to be edging away from the post-Vietnam era of soft complaints. Still lacking, however, is a workable substitute policy, perhaps one that denies to Havana and Moscow what they demand from the United States in trade and other joint undertakings.

That Carter policymakers are even approaching such a change of heart tells much about their learning process in the first year. Just possibly, it portends a larger share of realism in the second.