SOMETHING INCREDIBLE seems to be happening at the Rhodesia conference in London. The British, acting with a full Commonwealth mandate, keep gently but insistently forcing the issues that must be resolved to defuse the terrible wasting war in Britain's erstwhile colony. Multiracial government headed by Bishop Abel Muzorewa, desperate to win the international favor that alone will spare its supporters eventual ruin, keeps accepting the painful comprises proposed by the British foreign secretary, Lord Carrington. The guerrillas of the Patriotic Front, under heavy pressure from their sponsors in the front-line states to shift their struggle to the polls, keep squirming but remain at the table. There is, for the first time since the Rhodesian dispute began 14 years ago, light in the Rhodesian sky.

There are, formally, three issues. The first, the new constitution under which Zimbabwe would become legally independent under a fair democratic regime, has been resolved. The second is the transitional arrangements under which Britain would temporarily resume its authority and administer elections. The Salisbury government has accepted Britain's transition proposals and -- this is what the current tension in London is about -- the Patriotic Front is trying to secure certain adaptions of them. The third issue, due next, is a cease-fire. Each issue is harder that the last. But the further the parties go, the harder it is to turn back.

The Muzorewa government, urged on by South Africa, has bet everything on the conference. If the conference succeeds, Salisbury escapes from the war and gets its best available crack at power and stability. If it fails, Salisbury can count on a substantial and probably life-sustaining measure of British and American support, including the quick lifting of economic sanctions. All this might have been expected.

What was not expected was the approach of the Patriotic Front. The guerrillas went to London patently skeptical that the conference could serve their ends.To their own evident surprise, however, they have been drawn deep into a process of conciliation, and they now seem determined at least that if the conference fails it will not be because of them. To be sure, their friends as well as their enemies are pressing them hard. If they were having such an easy time of it in battle, their attitude might be contrary. Still, they, or their leaders, are earning fresh respect for the way they seem to be shedding the mystique of armed struggle and entering on the paths of compromise. Their objections to the British proposals on transitional security, for instance, are being presented in a spirit indicating that the differences, though large, can still be negotiated.

There is little for anyone else to do, the American government included, except to wish the conference well -- fervently and 24 hours a day.