Only a few weeks ago, Israel was in a state of near euphoria, thinking it was on the verge of achieving its most prized objective--the destruction of the Palestine Liberation Organization as a military threat and as a political force in the Middle East.

The government also believed it had the backing, or at least acquiescence, of the Reagan administration to accomplish these goals by military means.

Today, Israel seems to be watching, fettered in diplomatic irons, as the possibility of achieving these goals slips slowly from its grasp and the fate of the Palestinian guerrillas trapped in West Beirut becomes a well-publicized international drama.

"It appears that the grace period that Israel enjoyed and the opportunity to extract the maximum political benefit from operation 'Peace for Galilee' have come to their end," said the independent daily Maariv in an editorial yesterday.

In fact, the PLO, if anything, seems to be reaping unforeseen benefits from the Lebanese crisis, and Israeli officials admit they fear the Palestinians may yet snatch a political victory from their worst military defeat in Lebanon.

"They did score points and we know we are paying a price for this waiting," said one Israeli official briefing foreign correspondents here. "They are building up anti-Israeli feeling as well as an image of martyrs and of savage Israeli persecution."

In the past week, PLO political director Farouk Kaddoumi has been received for the first time by a British government minister, and today he met with French President Francois Mitterrand as part of an Arab League delegation touring Europe to discuss Lebanon.

Together with the comments of Secretary of State-designate George P. Shultz at his Senate confirmation hearings that "the legitimate needs and problems" of the Palestinians must be resolved "urgently and in all their dimensions," this diplomatic boosting of the PLO has made the Israelis officially "concerned and disappointed" about the trend of events.

"We are back to square one," the Israeli official said, in a comment meant to describe the state of the Beirut negotiations but reflective also of Israeli feelings about the whole crisis.

The changing attitude of Prime Minister Menachem Begin's government toward the Lebanese crisis is striking. It can be seen in the tone of statements by officials, particularly the Cabinet's well-known "hawks," and in the debate now in full swing among Israel's outspoken political commentators and newspapers.

Begin, bellicose and uncompromising in the early weeks of the invasion, is now cautioning Israelis to behave "with good sense and patience."

Defense Minister Ariel Sharon, the outspoken bullish architect of the Lebanese operation who until recently threatened a full-scale assault on West Beirut almost daily is for now talking about the merit of negotiations and the need to use "political means" to get the PLO guerrillas out of the city.

He told a parliamentary committee today that there were three considerations weighing against a military thrust into the city--casualties among Israeli troops, civilian casualties and the adverse effect it would have on Israel's relations with the United States.

Foreign Minister Yitzhak Shamir, after weeks of warning that time was running out on negotiations, said last night that Israel was "giving time and opportunity" to U.S. envoy Philp C. Habib to achieve the evacuation of the PLO guerrillas peacefully. His statement came after consultations with Prime Minister Begin and Sharon.

The Jerusalem Post reported that the government had decided to give Habib virtually all the time he wanted to resolve the outstanding issues in the complicated four-way negotiations .

The Israeli media reported this week that the key factor in the new "flexibility" of the Begin government was a stern letter from President Reagan last week. Reagan was quoted as warning Begin that a military assault on West Beirut would "grievously affect our bilateral relations."

That his letter has had a sobering impact on government thinking here was made clear by a comment yesterday by Communications Minister Mordecai Zippori, who said Israel, in deciding whether to enter West Beirut, had to consider "not only the military price but the price of antagonizing the United States . . . . This price is much higher than several weeks ago."

Other considerations, both foreign and domestic, weigh in the balance as well. The media today reported that the government was taking into account pressures from Egypt and the possibility that West European countries might impose sanctions on Israel if it carries out a full-scale assault on West Beirut.

Debate has resumed not only on the wisdom of entering Beirut but the whole rationale for pushing so far beyond the original objective of establishing a 25-mile-deep buffer zone in southern Lebanon.

The leftist Mapam party distributed a leaflet questioning the whole operation and charging that it was "based on the groundless assumption that it is possible to destroy the PLO by military means . . . . An operation intended to achieve this goal is an impossible mission."

Begin read the tract to the last Cabinet meeting and called on the attorney general to investigate whether a crime had been committed by Mapam in circulating the leaflet.

Yesterday it was reported that Abraham Burg, 27, son of Interior Minister Yosef Burg, was one of three reserve officers who met with Begin recently to discuss their opposition to the war and urge him to halt it.

A similar plea came today in the Labor confederation newspaper, Davar. Its editor in chief Hanna Zemer complained in an editorial of the "horribly simplistic" way of thinking of the government's leaders who thought military means alone could solve the Palestinian problem.

Military action "will not put an end to the Palestinian problem," she warned. "No action will put an end to terror. No action will solve any other problems we have." Zemer began her editorial with an emotional plea to Begin: "Don't send our boys into West Beirut."

The debate over Israel's objectives and rationale for being in Lebanon presently is focused on whether the Army should try to take Beirut. It is widely presumed that the Army would suffer heavy casualties if it tries to enter the the Western sector, and many Israelis are wondering whether it is really worth it.

Davar and the mass-circulation independent daily Yediot Aharonot both have compared Beirut to a "trap" with no easy exit for the Israelis.

"In our estimation, Beirut was a trap for us that we entered with closed eyes. Even if we get out of it one way or another, we will leave it worse off than if we had never gotten into it," said Davar.