IN TALKING ABOUT the Yemens, it is best to hedge. "Our" Yemenis (those supported by Saudi Arabia and the United States in Yemen proper) seem to be even more unstable in their politics and primitive in their economic development than "their" Yemenis (those the Soviets, Cubans and East Germans sponsor in South Yemen). So it would be rash, not to say unbecoming, for the Carter administration to claim that by its recent steps it has turned back South Yemen's border incursion against its neighbor and induced it to accept the Arab League mediation it had previously spurned. There is no call to trumpet an American "success."

The fact is, nonetheless, that the border is quieting down and the mediation is proceeding. And this happened hard on the heels of President Carter's decision to rush emergency military aid to the Yemenis and send a carrier task force to the Arabian Sea. Little of the aid seems actually to have been employed by the Yemenis, who need foreign help in operating any sort of modern equipment. But it seems fair to presume that the signal sent by the administration contributed to the decision by the South Yemenis and their Soviet patrons to divert Aden's feud with Yemen from a military track to a political one. The administration drew some congressional criticism for sending too much gear too hastily. But is it not possible that the impression thus cast of overdoing it helped make the policy work? If the truce sticks, there will be time to slow the flow.

It was only secondarily for the Soviet Union that this exercise in "drawing the line" -- the administration's own privately used phrase -- was conducted, and the Kremlin is presumably punching the sequence into its great-power computer.It was primarily for the notice of Saudi Arabia. And the Saudis are professing in their fashion a certain disappointment. They were only "mildly impressed" by American policy: "It took a long time to get some movement there and a lot of prodding." The Saudis are a tough crowd to reassure. Indeed, they are more Israeli than the Israelis in their persistent demand for reassurance and their equally persistent complaint that whatever reassurance is provided is not enough. Unlike the Israelis, however, they have a lot of oil. That should help assure them that the United States, not without a sigh, will keep on trying to please.