AMERICAN POLICY in Rhodesia is in a shambles. The effort, with Britain, to rope all the parties into a controlled transition to an elected majority-rule government has never seemed further from success. From time to time the mixed black-white interim government in Salisbury and its guerrilla challengers nod at the Anglo-American summons to an all-parties conference. But the offer has not had enough appeal - or force behind it - to cause either side to make the necessary adjustments. The internal people feel that an all-parties conference would merely receive their surrender. The guerrillas feel a conference would restrict the triumph they bid to gain in the battlefield. The "safety net" that its sponsors have always figured a conference to be seems now all but untied at the ends.

Perhaps events will turn around: A conference may yet help salvage something for those Rhodesians (conceivably a majority) who earlier seemed ready to cast their lot with the internal administration, and it may diminish the intensity of the civil war among guerrilla groups that seems likelier by the week. Perhaps. But if that does not come to pass, and even if it does, the United States must face up to its miscalculations. It is not just that American (and British) officials overestimated their capacity to perform a diplomatic mission akin to chaning a wheel on a speeding air: That is at least forgivable. They failed to seize the fleeting moment in which just conceivably the two sides had a roughly equal stake in experimenting with a political solution - the moment last March when the internal administration was formed. If, to even the odds, the United States had then shown a bit more favor for the internal effort, instead of feeling it had to accommodate the guerrillas' foreign patrons . . .

What now? Without illusion, Washington and London must hold open the door to an all-parties conference, and they must lend whatever impetus their fading influence allows to any negotiations the parties may still be tempted to try among themselves. Prompt and sharp protests against atrocities - protests uninhibited by political discretion - could have a humanitarian as well as a moral value. It might help to make sure the guerrillas understand that the United States, including the Congress, will inevitably be readier to deal with a moderate multiracial government sanctioned by popular vote than with a radical military gov ernment of a single race.

No one who has looked closely at the Rhodesian scene would argue that the future there was the United States' to mold. It is the local actors who, in their arrogance and ambition and fear, have made the birth of Zimbabwe tragic and wracking. Yet the United States and Britain have made their own separate contributions to this painful process, and although they do not suffer from it as the people of Rhodesia/Zimbabwe will and are, they have a responsibility, too.