The newest rationale behind Carter administration preference for Soviet-supplied guerrillas instead of pro-Western moderates to run Rhodesia is fear of yet another Cuban military operation, a rationale so clearly bogus that it brings the United States near a dead end in Africa.

While the British government gently nudges closer to supporting the internal (biracial) Rhodesian settlement, the official U.S. line grew tougher last week. The agreement between white Prime Minister Ian Smith and moderate black nationalists, a feat beyond imagination a short year ago, gives hope for a Western-oriented Zimbabwe (the future name for Rhodesia) with the white minority retaining a vital role. Yet the State Department's official spokesman on March 27 branded that settlement "illegal."

Since it seems clearly in U.S. interests, why should the Carter administration denounce it? The State Department's private answer: fear of Fidel Castro's potent Africa corps, fresh from military triumphs in Angola and the Horn of Africa. Should the United States promote a moderate government for Zimbabwe, policymakers told us, the Kremlin might signal Castro to move into Rhodesia and, considering the mood on Capitol Hill, Congress would permit no U.S. response.

Yet U.S. intelligence considers a Soviet-Cuban intervention most unlikely and in no way influenced by statements out of Washington. Those few sturdy souls at State seeking rationally in African policy recognize the Cuban threat as a ploy of policymakers headed by Andrew Young, ambassador to the United Nations.

Young's policy is geared entirely to the desires of Nigeria and the "frontline" states bordering Rhodesia. These nations have no interest in Rhodesia's white minority and are the patrons of Soviet-backed guerrillas in the Patriotic Front. Thus, the United States enters a policy dead end contrary to its own interests.

But support for the internal settlement is rising among the anti-communist black nations of French-speaking Africa (reflected by Gabon's March 24 statement expressing "optimism"). Once elections install black majority government with a black prime minister in independent Zimbabwe starting in 1978, world support would move toward the moderates. The one potential obstacle: a military takeover before then. That is easier said than done. Guerrilla activity is tepid. Base camps in Mozambique have been cut up by cross-border raids, and barely 1,000 guerrillas now operate within Rhodesia. So the Patriotic Front's only certain way of preventing elections would be intervention by Cuba's well-armed Africa corps.

But U.S. analysts regard such a move as most unlikely, partly because Moscow knows it would bring in the South African army (which, of course, needs no authorization from the U.S. Congress). Although outgunned and outnumbered, White South African troops handled the Cubans in Angola two years ago. This time they would attack in force.

What is truly involved in U.S. policy is not fear of Cubans so much as desire to please Nigeria and other black African states. That this is also British policy was pointed up in London during secret talks Feb. 20-23 between Foreign Secretary David Owen and the Rev. Ndabininghi Sithole, a principal black nationalist leader in the internal settlement.

While called that settlement "a step in the right direction," Owen repeatedly expressed his need to follow "the opinion of the world," adding: "We can't isolate ourselves from the rest of the world." By "world opinion," Owen clearly meant black African opinion.

Owen pleaded with Sithole to bring in Patriotic Front leader Joshua Nkomo, who exercises political control over the Soviet-equipped but military untested guerrilla army based in Zambia. Since Nkomo has little popular support inside Rhodesia, his lifelong dream of power depends on his army - not the ballot box.

On Feb. 20, Owen told Sithole that Nkomo must return for the internal settlement to be acceptable. On Feb. 21, Sithole replied with a question: "How do we get Joshua [Nkomo] to return when he says publicly he will destroy the polling booths in the country and disrupt the democratic process?" A Sithole lieutenant, Joseph Gopo, put it more graphically: "What choices are we given? Take Joshua as King or get slaughtered. He is welcome to try the slaughter." Sithole complained that "a space for Nkomo" means "not only Nkomo but the Cubans and Russians and Zambians as well."

Owen did not insist, declaring: "I don't rule out accepting the internal settlement." He seemed to accept Sithole's objections to an interim British high commissioner and guerrilla domination of Zimbabwe's new national army.

There is not much flexibility in Washington. A Sithole emissary visiting the State Department March 4 was informed that any settlement must be predicated on the Anglo-American proposals. When the official State Department spokesman was instructed on March 24 to put the "illegal" brand on the internal settlement, the United States reached a policy dead end, publicly rationalized by the Cuban spectre.