RHODESIA'S WHITE MINORITY announces that it is preparing to transfer power, at last, to the black majority. It has worked out an agreement with three of the country's black leaders, looking toward elections in which everyone over 18 could vote. Is that a real promise? Should you believe that the crucial decision has now been made? The unhappy history of Rhodesia invites a good deal of skepticism. Perhaps the best to be said in behalf of Prime Minister Ian Smith is that he is not, after all, making these concessions voluntarily. The country's economy is collapsing. Outward migration of whites is rising. The war is spreading, carried forward by guerrillas based beyond Mr. Smith's reach, in neighboring countries. The guerrillas' Patriotic Front is not much inclined toward bargaining, with Mr. Smith or anyone else. They tend to think, with Chairman Mao, that credibility proceeds from the muzzle of a gun.

This week's agreement leaves the guerrillas out. Britain and the United States had been pushing for quite a different kind of settlement, one intended to bring that faction in. The British-U.S. plan proposed that Mr. Smith return power to Britain, which would then prepared elections in which the guerrilla movement could take part. Otherwise, the British and American governments fear, the present war of insurgency against Mr. Smith's white government would turn itself into a civil war among competing black organizations. The disastrous example of Angola hangs heavily over their considerations.

But things have not been going terribly well for the British-U.S. plan. It isn't only that Mr. Smith has adamantly refused, from the beginning, to deal with the guerrillas. Last month British and American diplomats sat down with leaders of the Patriotic Front in Malta to try to draw them closer to a peaceful settlement. There doesn't seem to have been much progress. With the inconclusive end of the Malta talks two weeks ago, things suddenly began moving very rapidly in RHodesia, where Mr. Smith was dealing with the nonviolent black nationalist movement whose principal spokesman and symbol is Bishop Abel Muzorewa.

The guerrillas, from Zambia, predictably derided the agreement as a sellout. As for the British and U.S. governments, there seems to be a curious difference in attitude emerging between them. Both of them emphasized that it is impossible to make a judgment until the full terms of the agreement appear. But where the British indicate support for anything that looks likely to bring peace, the American position suggests an increasingly emotional commitment to a political role for the guerrillas. Britain's foreign secretary, David Owen, acknowledged that the settlement was a significant event and was to be welcomed. But the U.S. ambassador to the United Nations, Andrew Young, denounced it sharply: "This is not a settlement addressing itself to the issues for which some 40,000 guerrillas are fighting."

The American responsibility is not to the guerrillas but to a peaceful settlement. Perhaps it cannot be reached without them, as Mr. Young believes. But the outcome of the Malta talks seemed to suggest that the guerrillas are not much interested in negotiation. Now Prime Minister Smith has taken another course. Is his bargain with Bishop Muzowera genuine, or a fake? The test will be whether, in the transition about to begin, actual authority in the security forces. If the army and police remain firmly under the present white control, that will be a very bad sign, and any election results will be suspect. The burden is still on Mr. Smith to demonstrate that he is dealing in good faith.