NO SOONER had Menahem Begin promised Jimmy Carter personally last July that Israel would restrict new settlements on occupied territories than the Israeli government legalized three existing but previously unauthorized settlements in the West Bank. Only three weeks later three new civilian settlements were established. Responding to American insistence that such settlements were illegal and an obstacle to peace, Foreign Minister Moshe Dayan is September assured Jimmy Carter that there would be no more settlements except within existing military camps; civilian settlements, it was agreed, were more provocative, for seeming more permanent.

Then it turned out that the Dayan pledge was good only for a year, or so some American officials understood. "A" year soon became "the" year, 1977. On Jan. 3, 1978, some weeks after Anwar Sadat's Jerusalem initiative transformed the diplomatic landscape, the governments - albeit inside military perimeters - in a part of the West Bank heavily populated by Arabs. Just the other day American officials detected signs of yet another new settlement. Mr. Carter said he'd been assured it was only an archaeological dig, but the people living there say they intend to stay.

What is going on? Many Israelis, even some within the government coalition, are shocked to find Mr. Begin pursing a policy so provocative and devious. A policy of sneaking new settlements in between the lines of assurances to the United States is offensive to the United State, and to Jimmy Carter personally. This is also a matter of no small importance in maintaining the mutual American-Israeli confidence that is vital, or so we have always though, in Israeli political calculations. Moreover, Israel cannot dream of persuading already skeptical Arabs that it is serious about peace if, at an immensely fragile moment, it acts in the old spirit of defying Arab nationalism by "creating facts" - asserting Israeli control in gray areas by establishing new settlements. True, Mr. Begin is acting in a way consistent with his own peace proposal, which calls for continued Israeli settlement in a West Bank accorded "self-rule." But this utterly ignores that the Arabs have not accepted the Israeli proposal. Indeed, negotiations on it have not even begun.

There is bound to be an uproar, in Washington and in Cairo, over the emerging shape of Israel's settlements policy. But the main place where there needs to be an uproar is Jerusalem. The Israelis, as we see it, are still in the process of forming a national consensus on how to treat the new opportunity provided by Anwar Sadat. Some, including some in the government, evidently feel that they can reap the benefits of peace without materially sacrificing the comforts they have drawn from possession of Arab land for the last 11 year. This seems to be the spirit in which policy on settlements has been made. We have no wish to prejudice the outcome of whatever Arab-Israeli negotiations may yet take place. But we are increasingly doubtful that Israelis can have the settlements, and a settlement too. They must choose.