The return of Palestine Liberation Organization fighters to take part in the current battles in Lebanon's Shouf mountains is deeply upsetting and embarrassing to Israeli military officials.
Israel's expulsion of the PLO from Beirut a year ago was viewed as the greatest triumph of its June 1982 invasion of Lebanon.
Behind the scenes, Syrian participation in the Lebanese civil conflict is also bothering the Israeli defense establishment. But until now the policy of Israel - whose withdrawal of troops from the Shouf last week set off the fighting between rival Lebanese forces seeking to control it - is to forgo any military intervention in favor of intense diplomatic activity to try to find a peaceful solution.
And Israel appears to be counting heavily on its repeated warnings to the Druzes - the secretive Islamic sect that predominates in the Shouf and is battling against Christian Phalangist militiamen - to persuade them not to let PLO fighters stay there once the battles are over.
The Druzes appear on the verge of total victory as the Phalangists have withdrawn from all their strongholds in the mountainous Shouf region except Deir al Qamar, which Red Cross supply convoys finally reached Monday to deliver food to 25,000 trapped Christian refugees.
Israeli military sources say about 1,700 PLO fighters from several different member organizations have joined the Druzes. They arrived from east Lebanon's Bekaa Valley, now under Syrian control, where they had re-established bases after Israel drove them out of Beirut and southern Lebanon.
The Druzes, longtime allies of the PLO, are fighting to push out the Phalangists from the Shouf, which they entered on the heels of Israeli invasion troops. The Druzes are also seeking a bigger slice of Lebanese political power, which is apportioned by religious sect and is disproportionately weighted in favor of the minority Christians.
The Druzes entered into open political alliance with the Syrians to give them leverage against the Lebanese government and the Christians.
Israeli sources believe Damascus is certain to gain from events in the Shouf. But they say the Syrians are also sensitive to blunt Israeli warnings about an open Syrian role and as such are playing a careful game. According to an Israeli military source: ''There is no direct Syrian military involvement until now in the actual fighting. Artillery support, logistical support, yes, but no indigenous Syrian units in the area as far as we can ascertain.''
But the Israelis believe the Syrians are directing operations from behind the scenes.
Israeli sources say they have ''grounds to hope and believe'' that PLO re-infiltration will not be the harbinger of a more massive return of PLO units to central Lebanon or even to Beirut itself.
The key basis for this hope seems to be Israeli contact with the Druzes.
Israeli official sources say a PLO return is ''not in the Druzes' interest.'' They indicate Israel has delivered stern warnings to the Druzes not to allow the PLO back and has received certain undertakings from the Druzes in return.
Such reliance on the Druzes reflects a sharp shift in Israeli policy away from their one-time close allies, the Lebanese Maronite Christians.
Under the stewardship of former Defense Minister Ariel Sharon, Israel banked on the Maronites to create and control a strong central Lebanese government. For this reason Israel allowed Phalangist militiamen into the Shouf, upsetting the delicate power balance there. This thinking has been reversed. New Defense Minister Moshe Arens has concluded that the Christians are not strong enough to dominate, and they must compromise with - as Israel must deal with - the numerically superior Muslim factions.
Mr. Arens has expressed regret that the Phalangists were ever allowed into the Shouf. In a sharp exchange with Mr. Sharon at a Cabinet meeting on Sunday he said, ''The great (mistake) Israel made was to send the Phalangists into the Shouf.''
Israel tried unsuccessfully to mediate between the Druzes and the central Lebanese government in order to gain Druze permission for the Lebanese Army to take up positions in the Shouf which Israel had abandoned. Israeli officials say their attempts failed because the Druzes distrusted the government as too pro-Christian, and the government was too unwilling to compromise with Islamic minorities.