THE RHODESIA-ZIMBABWE negotiations have produced a marvelous success, and at exactly the right moment -- when everything else seems to be going to hell in a handbasket. Within weeks of winning agreement on a new constitution, Britain has gotten the Salisbury government and the Patriotic Front to agree on the arrangements for transition from war to elections. It remains only to get the multiracial regime of Bishop Abel Muzorewa and the guerrilla forces led by Robert Mugabe and Joshua Nkomo to work out a cease-fire. This presents vast practical difficulties, but there is broad confidence it can be done.

How did it happen? How did Prime Minister Margaret Thatcher and her foreign secretary, Lord Carrington, arriving on a scene strewn with the failures of their predecessors over 14 years, succeed in ending the white minority's rebellion in the erstwhile British colony and in creating the conditions for authentic majority rule? Two factors were crucial. First, the moment was right. Each side had been hurt enough to have lost any false confidence in military victory and to realize that its better prospect lay in going over to political competition. It took some 20,000 lives (equivalent to nearly 1.5 million on an American scale) and a terrible economic and social toll to produce this common maturity.

Then, the Thatcher-Carrington combination was dynamite. The no-nonsense Mrs. Thatcher, leader of a party long tilted to the Salisbury side, bravely retreated to the middle ground after her election, and drew Salisbury and the Front into a last-chance negotiation that split their differences down the middle. Lord Carrington devised brilliant tactics -- seizing on the fortuitous meeting of the British Commonwealth in Lusaka last August to win a broad negotiating mandate, playing on Salisbury's determination to win favor in London (and Washington) and on the Front's determination not to be left behind, making himself the sole arbiter of the agenda and the pace of the talks.

Whether a Labor government in London, working with a U.S. administration with Andrew Young at the Africa helm, could have pulled off a Rhodesia settlement is, on the record, dubious. The Labor-Young combination looked too pro-guerrilla to gain a clear commitment to negotiations from either Salisbury or the Patriotic Front. Yet the idea of a negotiated solution was sound, as the Conservatives' success with it indicates. And a settlement, even one made by other hands, will be of immense benefit to the United States. A settlement in Rhodesia, which will almost certainly be followed by one in Namibia, will all but foreclose a direct Cuban-Soviet involvment and an ever-wider war in southern Africa. It will strengthen the forces of peaceable change in South Africa. It is sensational, and all those who are helping to make it real deserve the congratulations and therespect and, where feasible, the assistance of the rest of us.