RHODESIA HAS done something seemingly without precedent: stopped a long, bitter civil war short of military victory by one side or the other, and converted it into an agreed electoral solution. Civil wars, you recall, are supposed to have at their core that extra element of brotherly hate imcompatible with such a solution. Who can think of another? But it's happened in Salisbury.

What happened is this: the British, experts at failure in Rhodesia, took the occasion of Margaret Thatcher's own election lst year to make one more go. It was, in retrospect, the magic moment when the weariness and determination of both sides were in rare poise, ready to be guided by a sure outside hand. Once the process of accommodation got under way, there was no stopping it. The radical guerrilla leader Robert Mguabe was unquestionably Britain's least favorite candidate. Through the election run-up, Mr. Mugabe acted as though the British were trying to snatch victory from him. But the British held the ring and the Mugabe orces, and others, fought in it -- amazingly fairly, all things considered. Mr. Mugabe, with his resistance record (and his jousting with the British during the electoral campaign) and his tribal political base, carried the day.

Given the circumstances, it is probably a good thing that Mr. Maguabe won big. Had he lost, he might have been sorely tempted to keep fighting. Had he done the expected and won only a plurality, political strife fading into irregular war would have gone on indefinitely. But with a mandate his responsibility is clearly fixed, and he has the opportunity to play the statesman. His gestures yesterday to Rhodesia's jittery whites, asking the British governor to stay on for the transition and inviting the white commander to lead the new integrated defense forces, were ressuring.

The specter now haunting Rhodesia is the white flight, economic collapse and black civil war that attended the collapse of Portugal's African empire. To avoid an emulation of tht model in southern Africa has always been the goal of outside powers who wanted to see decoloization there wind down peacefully. Mr. Mugabe is a Marxist who believes in a single vanguard political party and a vaguely socialist planned economy. He now becomes the elected prime minister in a lively multiparty system with one of Africa's more productive -- non-socialist -- economies. The more dogmatically he tries to put the two broad features of his program into effect, the more certainly Zimbabwe will falter.