THE PATRIOTIC FRONT in Rhodesia has done what a lot of people, friends as well as foes, doubted it would ever do: abandon the battlefield for a political contest that will surely try its fragile unity and perhaps deny it power. To be sure, the Front had reason. Its nearly exhausted sponsors in the front-line states were insisting on it. Its rivals in Salisbury, having pretty well consolidated their military operations, were moving toward breaking their longtime international isolation. Still, the most fateful thing in a war, after starting it, is recognizing when to stop it. For making this choice the Front deserves immense respect -- no less, in fact, than the Salisbury regime deserves for the earlier political compromises that alone let the British extract matching compromises from the Front.

What Salisbury and the Front have done is commit themselves to building a functioning black-majority, multiracial democracy in southern Africa: a miracle. There is no underestimating the obstacles yet to be overcome, but there is also no underestimating the importance of the effort. The success that Prime Minister Margaret Thatcher's policy has achieved in Rhodesia should facilitate a paralled success for Western diplomacy soon in the neighboring territory of Namibia. Not only will this double triumph preempt a larger continuing southern African war, one that alway certainly would have drawn ever-greater Cuban and Soviet participation. A fresh and near example of racial cooperation is bound to play usefully in the intense debate South Africa's ruling whites are conducting on their own country's future.

In less than half a year's time Mrs. Thatcher turned a sow's ear of a negotiation into a silk purse. She has reaped the credit during her current Washington visit -- and deservedly so. It's only fair to note, however, the American contribution. Jimmy Carter was himself never able to become an efective mediator in Rhodesia. But he was correct in contending that there was a peaceful, pro-Western, multiracial way -- that black animus, Marxist ideology and Soviet-Cuban strategy did not have to be controlling. In the last lap, too, he rose above the pro-guerrilla cast that had long dogged his diplomacy, resisting an effort to have him undermine Britain's down-the-middle approach by keeping sanctions on alone. The United States thus comes out in a good position to play its part in helping Rhodesia become Zimbabwe, a country all of its citizens can call their own.