Mr. Begin's opportunity

With Ariel Sharon no longer his defense minister, Prime Minister Begin has an opportunity to speed up the negotiations on a withdrawal of foreign forces from Lebanon. By doing so, he can help end the anguish of the people of Israel, with whom the war in Lebanon was never popular. He can also avoid embittering relations with the United States, which grows increasingly frustrated over the lack of diplomatic progress.

It was Mr. Sharon who largely orchestrated the Israeli invasion of Lebanon and Mr. Sharon who has been most determined to keep Israeli forces there. Although the feisty warrior may continue to play a role, his stature and influence in the wake of the report on the Beirut massacre have diminished. That provides an opening for Mr. Begin to bring political rather than largely military judgments to bear on the issue of Lebanon. It will now be seen whether his government indeed wants peace in the Middle East, or is simply laying the seeds for continued strife.

Time is of the essence, because without an agreement on a Lebanese pullout the prospects for drawing King Hussein into the negotiations over West Bank autonomy may evaporate. And if the Jordanian monarch, who insists on a sign of Israeli good faith, does not make his move soon, the United States plan for resolving the Palestinian question and achieving an Arab-Israeli peace will be aborted.Israeli colonization of the West Bank continues apace, and it is a question how much longer the Reagan initiative - or any other peace plan - will remain relevant.

This is why the US has begun to toughen its stance of late. President Reagan has pointedly urged Israel not to become an ''occupying force'' in Lebanon and Defense Secretary Caspar Weinberger has refused to accept Israel's offer to share military intelligence from Lebanon with Washington. Such moves are tempered with public assurances that the US will not try to arm-twist Israel by economic or other sanctions. But the White House clearly wants to show its impatience. Mr. Begin cannot rule out a confrontation with Washington if there is not progress soon.

To be sure, there is a political risk in this for President Reagan. It is hard to ''take on'' Israel at any time. It would be doubly hard in the charged atmosphere of an election campaign, which is already underway. But it is also true that Mr. Reagan, weakened by the midterm elections, is vitally in need of a success or two if his party is to emerge victorious in 1984. Israel should not count out the President's ability to resist pro-Israeli political pressures at home in the interests of achieving a Middle East peace - an accomplishment which would certainly have the backing of the American people at large and stand him in good stead politically.

However, Israel's full withdrawal from Lebanon should come not because the United States wants it but because it would be morally right and politically healing. The demonstrations and violence triggered in Jerusalem as the Cabinet debated Mr. Sharon's ouster bore testimony to the deep divisions now rending Israel. They were a reminder of the political and moral toll which the invasion of Lebanon has extracted, and of the urgency of negotiating a security agreement and, together with Syrian and remaining PLO forces, leaving that country. The fact that Mr. Sharon's political goal in Lebanon has not been realized - namely, establishment of a Christian Phalange puppet regime in Beirut closely allied with Israel - should be further incentive for such a withdrawal.

Israel has rekindled the respect of the world by demonstrating its capacity for democracy and rule of law. The Cabinet's castigation of Mr. Sharon - by a vote of 16 to 1 (he being the only dissenter) - shows the depth of conscience of which Israeli society is capable. The need now is to summon up that conscience in the service of a lasting peace.