FOR THE SECOND TIME in a fortnight, pressures induced by American diplomacy in a crisis area have helped provoke an unexpected but welcome local initiative. First it was the Mideast, now Rhodesia. For 12 years, since he seized power for the quarter-million whites, Ian Smith has refused to share power with six million blacks. Now, finally, he says he's accepted the principle of one man, one vote. The fine print is not yet public. Specifically, are the guarantees for whites that he demands from the "internal" black political parties, in return for offering them one man, one vote, within the capacity of self-respecting blacks to give? This will determine whether Mr. Smith is simply trying to play off "internal" black against the "external" guerrilla leader, Joshua Nkomo, with whom he's recently been secretly dealing. Meanwhile, he deserves a cautious cheer.

Publicly at least, the guerrillas surely will curse the deal: It cuts them out. One must understand about the guerrillas, however, that even if they were united they couldn't win in a fair election. This is their judgement: It's why they would not accept the Anglo-American plan, which stipulated elections. (Mr. Smith's complaint was that the plan did not offer adequate guarantees to whites.) Already most members of the Smith army and police are black. Under majority rule, black resistance to guerrillas presumably would grow. Probably some guerrillas would drift home and become civilians. Others, unwilling to lose the power they believe they earned, doubtless would fight on.

Showing a large-mindedness not altogether evident in respect to the Mideast, the United States has said, in regard to the local initiative in Rhodesia, that it's useful to get Ian Smith on record for one man, one vote. "Zimbabwe" could become the only state in Africa ruled by that principle. But while American diplomacy has aimed at securing majority rule, it has also aimed at accommodating Rhodesia's "front-line" neighboring states, by offering a chance at political power to the guerrillas they sponsor and be trying to forestall a post-independence war in Zimbabwe - a war that could upset the whole region. There's the rub.

Officials are prudent to anticipate the postwar scene. One reason why the Anglo-American plan could yet come to attract the parties is the contribution it could make to regional peace. it could also happen, however, that a continuing war might come precisely from guerrillas supported by the countries that now profess to worry about such war. If it comes to that, there should be no question of where the United States' moral and political loyalties lie: on the side of the government freely chosen by the people of Zimbabwe.