A year ago President Reagan launched a bold initiative for peace in the Middle East. The Sept. 1 plan, which called for Palestinian self-rule in the West Bank, raised hopes that the United States was again prepared to play a vital role in regional peacemaking. Today that plan is all but forgotten. With conditions in the area not conducive for negotiation, and with a sensitive election looming next year, - and now with Menachem Begin's announcement that he would resign - the President would seem to have a good pretext for letting the whole matter drop.
But would that serve the ultimate cause of peace - a cause to which the United States is bound to return?
At the moment US diplomacy is focused on Lebanon and on assuring a coordination of forces as the Israelis withdraw southward to the Awali River - a narrow focus which in itself points up just how derailed the American initiative of a year ago is. The partial Israeli withdrawal suggests that Syria and Israel will now be ensconced in Lebanon for the indefinite future.
Beyond Lebanon, however, lies the larger deadlocked question of Palestinian self-determination. The Begin government shrewdly used this period to accelerate its colonization of the West Bank. If there were any doubt at all that Israel seeks to incorporate these occupied lands and resist a territorial compromise such as that proposed by Washington, it was dispelled by Israeli Defense Minister Moshe Arens in remarks recently. ''We're approaching the point where the facts are irreversible and our control over Judea and Samaria is assured,'' he told members of his party. Although Mr. Arens has adopted a softer more flexible style in the West Bank, he clearly shares Mr. Begin's expansionist aims.
Unfortunately, the US position on Israel's settlement policy has become increasingly blurred. Ever since the 1967 war the US has characterized the Jewish settlements as ''illegal'' and firmly supported UN Resolution 242 calling for withdrawal of Israel from Arab territory. The Reagan peace plan of Sept. 1 also reaffirms 242. Yet recently the State Department called a dismantling of existing settlements ''impractical'' - a gratuitous comment that can only encourage Israeli expansionism.
In an apparent effort to minimize the damage of that diplomatic gaffe, Mr. Reagan this weekend reiterated his commitment to his peace plan. He called the establishment of new settlements in the West Bank "an obstacle to peace." But such words, to have meaning, must be matched by an active, full-time US diplomatic effort aimed at achieving a peace settlement.
That is not the case. In fact, US policy now seems to be slipping back from what had become a more balanced approach in the Middle East. The retreat is regrettable, given the fact that the US alone is able to be an arbiter of peace in the region.
The Israeli creation of ''irreversible facts'' continues apace. There now are 30,000 Jewish settlers in the West Bank. The Israeli government has plans to boost this number to 100,000 by 1986, and the World Zionist Organization would like to see more than one million Jews living there by the year 2010.
What such absorption of the West Bank would mean for the character and security of Israel is the subject of intense debate among Israelis. It should also be the concern of the US government. President Reagan may calculate that the US need not worry at the moment, given the state of disarray in the Arab world and the new uncertainties in Israel. But, if the history of the region is any guide, the Palestinian question will not stay dormant. If the US is to forestall future conflict, it must stick by its principles - including the principles enunciated in the Sept. 1 peace plan.
It must also find the political will to pursue peacemaking with far greater foresight and determination than it has displayed to date.