When Israel first broke the terms of the Habib plan by inching its Army forward in West Beirut's southern suburb of Bir Hasan Sept. 3, U.S. officials chose not to make much of the incident.

Two days earlier, the Palestine Liberation Organization completed the withdrawal of its guerrillas from the Lebanese capital under the accord hammered out over the summer by special U.S. envoy Philip C. Habib, and euphoria was running high among confident U.S. officials. Now, following the assassination of president-elect Bashir Gemayel, Israel's occupation of Moslem West Beirut and the subsequent massacre of Palestinian civilians in the Shatila refugee camp by Israel's Christian allies, Western officials and Arab analysts here agree that the incident in Bir Hasan signaled a critical shift in Israeli policy.

These analysts say Bir Hasan was not just a violation of the painstakingly negotiated Habib plan, but an indication of Israel's disregard of previous commitments under it. They now see it as Israel's first concrete move in a campaign to bury President Reagan's new Middle East initiative in the shifting sands of Lebanon.

The move into Bir Hasan came just three days after Reagan proposed, and Israel strongly rejected, a plan for placing the Israeli-occupied West Bank and Gaza Strip once again under the rule of Jordan. The move at Bir Hasan was justified by Israel at the time as necessary to protect mine-clearing operations by the Lebanese Army and the multinational force along what had been the PLO's southern perimeter in West Beirut. But analysts now see it as preparation for last week's larger invasion of West Beirut -- that move justified by the Israelis as necessary to keep peace following Gemayel's assassination.

Western officials with a long history of dealing with the Israelis say that, as it has so often in the past, Israel was seeking with its series of moves into West Beirut to create new facts on the ground in Lebanon that would preoccupy the attentions of Washington and world leaders away from a Middle East peace package.

"What Israel has done by ripping up the Habib accords and occupying the West Beirut is to create a whole new set of problems that we thought we had already resolved," said one senior Western official here, who did not want either his identity or nationality revealed. "We are now going to have to go into a period where not only will the United States have to begin renegotiating much of what had already been negotiated and resolved under the Habib plan, but a whole new set of issues has been created that will tie up everyone's attention and energy for months to come, if not years."

To officials involved in the negotiations, there is a great sense of deja vu. Instead of the beginning of talks about an overall settlement of Middle East problems, there are once more discussions and disagreements about the duties and responsibilities of the multinational forces being sent anew by the United States, France and Italy.

In a repeat of the earlier Habib negotiations, every inch of ground to be given up by Israel, and the conditions for each withdrawal, has to be argued. The purpose: to get Israel to return to the positions it occupied Aug. 12, when the initial cease-fire around Beirut was imposed after the Israeli and Lebanese Cabinets approved the Habib plan.

"It is not a question of being back to square one again," said a senior Western diplomat. "We are at minus three or four now. We have a whole new series of negotiations we must conduct just to get back to the point that we were at before. Then we will have to begin renegotiating what we already had agreed upon last August."

After that, there will still have to be discussions about Israel's withdrawal from the Beirut airport. The larger negotiations for a mutual Israeli, Syrian and Palestinian withdrawal from all the rest of Lebanon will then have to be tackled. According to dejected officials here, all that will take months, if not years.

Support for that assessment came from the movements of Habib, who had returned to Lebanon this week for the inaugural today of President Amin Gemayel. He was suddenly plunged into feverish negotiations that took him to Jerusalem this afternoon to arrange for docking here of the returning French units of the multinational force.

The French remained afloat today in the Mediterranean Sea outside Beirut harbor, pending the results of Habib's intercession to achieve Israeli evacuation from the port so they can land.

Extended negotiations, officials here say, are just what Israel wants -- explaining why Israel has seemingly acted so recklessly since Aug. 31, when Begin received a letter from Reagan announcing his new initiative.

Senior Western officials close to Habib's resumed mission privately admit that Israel's attitude toward withdrawing from around Beirut changed overnight with introduction of the Reagan plan.

Bir Hasan, in this view, was just the first Israeli salvo in a battle to force U.S. and world attention back on Lebanon so the more critical issue for Israel -- its creeping annexation of the West Bank and Gaza Strip -- would be blurred or forgotten.

In the sudden Israeli advance into Bir Hasan, mines previously laid by the PLO were cleared to open the way for the quick advance of Israel's armor into the city's soft southern underbelly, including its now infamous refugee camp of Shatila, just to the northeast of Bir Hasan. It also gave Israel a valuable observation post atop a nearby seven-story Lebanese officers' residency apartment.

Despite Israeli claims that its all-out invasion of West Beirut last week was necessary to preserve order following Gemayel's assassination, there has been more, rather than less, disorder and bloodshed.

Prior to the Israeli occupation, normality had been returning to the city gradually: local leftist militiamen had been pulling back as a result of a "security plan" imposed by Prime Minister Shafiq Wazzan, public services long disrupted by Israel's summer-long siege of the city were being restored and even the mounds of mouldering garbage and rubble were being picked up.