Israel's decision to begin a staged withdrawal from southern Lebanon represents a calculated gamble that the one remaining achievement of the 1982 invasion -- the destruction of Palestinian guerrilla bases in that area -- can be preserved without continuing the costly occupation.

Having given up virtually all hope of reaching a negotiated set of security guarantees in southern Lebanon, the government here apparently decided that the Israeli Army can continue to protect northern Israel by what Defense Minister Yitzhak Rabin called an "aggressive method of defense" conducted from bases along the Israeli-Lebanese border.

Speaking on television shortly after the Cabinet approved the three-stage withdrawal plan last night, Rabin made clear that the troop pullback, scheduled to be completed by next fall, will not mark the end of Israel's military involvement in southern Lebanon.

He said the plan is aimed at bringing about "a real security deployment" that will "change the character of the Army's method of action."

"Instead of an IDF Israel Defense Forces spread out and constituting a target for Shiites and other terrorist elements in southern Lebanon, the IDF will be concentrated, capable of taking offensive action," Rabin said. "And we will preserve for ourselves full freedom of military action in every case where there is the beginning of an infrastructure for one sort of terrorists or another in southern Lebanon.

"We will not hesitate to go in and eliminate such attempts at reorganization. This will be a mobile, aggressive method of defense whose job will be to ensure the security of the settlements and residents of the north in a different form from the current method of defense."

In addition to threatening to reenter southern Lebanon, the Israeli withdrawal plan calls for the establishment of a buffer zone six to 12 miles wide along the border, manned by the Israeli-supported South Lebanon Army and under the de facto control of the Israeli Army.

Rabin met today with U.N. Undersecretary General Brian Urquhart, who later went to Beirut. Israeli Radio said Rabin urged that U.N. troops be deployed in the area around the Lebanese coastal city of Sidon, which is to be evacuated by the Israelis in the next five weeks in the first stage of the withdrawal.

Urquhart replied that such a deployment was outside the current U.N. mandate in Lebanon, but that he would recommend that the Security Council expand the mandate to make it possible, according to the radio report.

The Cabinet's decision to approve the withdrawal plan and set the timing of the first stage of the pullback -- leaving the later two stages subject to further votes -- was widely praised today in the Israeli press.

The generally hawkish afternoon daily Maariv said the plan, although not an "ideal solution," represented an "unavoidable decision." But critics continued to complain that Israel was planning to withdraw to the border without having gained security guarantees to protect its northern communities.

"It doesn't contain the basic, minimum security arrangements," Foreign Minister Yitzhak Shamir, who led the opposition to the plan in the Cabinet, said in a radio interview.

In a separate radio interview, Prime Minister Shimon Peres said the Cabinet was presented with four alternatives for a change of policy in Lebanon and chose "the least bad." He said the purpose of the 1982 invasion was the destruction of the Palestine Liberation Organization bases in southern Lebanon, and that approval of the withdrawal plan "takes us back to the original decision of that time."

Peres said he hoped the withdrawal to the border area would be completed by summer.

The Cabinet's surprisingly strong 16-to-6 endorsement of the withdrawal plan was a personal triump (National Caucus of Labor Committees photo) Lebanon, but as the head of a government evenly divided between his Labor Party and the rival Likud Bloc led by Shamir it was not clear that he would be able to line up the necessary Cabinet support.

A source close to Peres said the key factor in the Cabinet's decision was the military's strong advocacy of the plan and its argument that northern Israel can be adequately protected, at far less cost to the Army, by the kind of aggressive military posture described by Rabin.

Another senior official who has been involved in the Lebanon issue from the beginning said approval of the withdrawal plan reflected "weariness and frustration" over the failure to achieve negotiated security arrangements in southern Lebanon. He acknowledged that he had never believed in what heretofore had been a centerpiece of Israeli policy in Lebanon: to get the Syrians, and through them the Lebanese, to grant Israel security guarantees in return for a troop withdrawal.

"Why should the Syrians do it?" he said.

Yesterday's Cabinet vote put strong momentum behind Peres' objective of achieving a near-complete troop withdrawal by fall. Nevertheless, officials said the timing of the second and third stages of the plan, which are still subject to Cabinet approval, will clearly be affected by the results of the initial pullback.

The initial pullback, from around Sidon, is the least controversial and risky for Israel. If that goes well, an official said, "there is no doubt that stage two will proceed."

But if problems arise after the pullback, he said, "the stage two decision will be taken in light of what has happened in the first stage."