How long will US military stay in Lebanon civil war?

This may only be the beginning of American involvement in Lebanon, even if a cease-fire is achieved in the country's fierce civil war. But if the mountain fighting goes on much longer, United States assistance may not be able to prevent the Lebanese Army and the Gemayel government from disintegrating.

The complex skein of international and domestic factors which have caused this fighting and ensnared the Americans will start to unravel if the parties stop shooting and begin to talk.

Even as US naval guns pound the Shouf mountains near Beirut, US diplomatic efforts have centered on reaching a cease-fire. President Reagan warned on Wednesday that if these efforts failed the ''peace plan for the whole Middle East . . . I think also goes.''

The dragging search for a cease-fire being conducted by the Saudi mediator, Prince Bandar ibn Sultan, and US special envoy Robert C. McFarlane seems to have eliminated most of the original obstacles. Despite early objections, the Druzes now fighting the Lebanese Army have agreed that the Army can retain the positions it has already taken in the mountains following a cease-fire.

A committee of all the warring factions would then work out security arrangements for the mountain area and a Lebanese reconciliation conference with Syrian and Saudi observers would try to find a new formula for power sharing. It is hoped the conference would help to dampen the bitter intercommunal feuds between Druze and Christian militiamen which underlie the current fighting.

The Syrians, however, still seem interested in prolonging the suspense. The 40,000 Syrian soldiers in Lebanon and the the Syrian alliance with the Lebanese Druzes gives Syria a key role in the negotiations. The Syrians are demanding that two prominent Lebanese Muslim leaders be excluded from the reconcilation conference. One of the leaders is Prime Minister Shafik Wazzan, whom the Syrians and some Muslims dislike because of his closeness to Christian President Amin Gemayel.

The Lebanese government so far is trying to defend its sovereignty by insisting that these men attend. And even Syria's ally, Druze leader Walid Jumblatt, is making a much-noticed show of independence by being more conciliatory on this issue.

But the Syrian stubbornness provides a warning that even if a cease-fire is finally reached, this may be only the beginning of protracted and complex negotiations. The failure of negotiations could spell new fighting and new pressures for continued US military involvement in support of the Lebanese.

Noted one Western diplomat, ''This won't be a cease-fire which will let everyone go out and picnic in the Shouf. There will be constant probing and pushing by all sides.''

The first hurdle will be trying to get representatives of the hostile parties to sit and find a formula for security in the Shouf region. The talks might be eased if an international force could police the Shouf. But so far the four members of the multinational peacekeeping force in Beirut have been hesitant to formally enter the Shouf. A Soviet veto would likely prevent the dispatching of United Nations forces there.

An even more daunting task will be reconciling the Lebanese communities. Just getting the Lebanese leaders to one table would be a triumph.

For example, the venue of a reconciliation conference will be Saudi Arabia because Druze leader Walid Jumblatt fears returning to Beirut lest the Lebanese Forces - whose political wing the Lebanese Front will be represented at the conference - might assassinate him. And one key Christian Maronite would-be participant, Suleiman Franjieh, holds another, Pierre Gemayel, the father of the president, responsible for the assassination of his son, Tony Franjieh.

A key political bloc, the National Salvation Front, which includes Messrs. Jumblatt, Franjieh, and former Prime Minister Rashid Karami, has already pledged to demand the rejection of the May 17 accord between Lebanon and Israel. But President Gemayel fears that if he jettisons the accord, the Israelis will remain indefinitely in south Lebanon, leaving the Syrians ensconced in east Lebanon.

As if such obstacles were not enough, the conference faces the problem of power-sharing which has bedeviled Lebanon for the past 40 years. A 1943 forumula apportions political posts by religious confession: the growing Muslim communities including the Druzes and the Shiites want more power which means the Christians must accept less.

If diplomatic efforts fail, the US will once again have to reassess how far it can go to support the Lebanese Army. US sources admit, despite the official explanation that the Americans use force only to defend their personnel, that USnaval guns this week were used to rescue the Lebanese Army from potential defeat at the besieged Shouf town of Souk al Gharb.

They also say bluntly that because of its command of Beirut, the Lebanese presidential palace, and the defense ministry, the US military will not let Souk al Gharb fall to Druze forces. But the Lebanese Army has been pressed hard over the past two weeks, and even if refreshed by a cease-fire, the Army would be hard put to fight new battles unless it had several months to increase its strength.

Moreover, if a cease-fire broke down, the Druze, who now insist that any Palestinian supporters are playing only a secondary role, might carry out pledges to invite them in.

Israeli Foreign Minister Yitzhak Shamir pledged that Israel would not intervene again in the Lebanese mountains. And the Lebanese Forces, or Phalangists, who were mostly driven out of the mountains by the Druzes have been severely weakened as a fighting force.

One informed Lebanese commented, ''As long as Israel doesn't intervene in the Shouf, it is a Syrian area here. That is, unless the US is willing to send Marines into the Shouf.''

But the terms of new US legislation being readied to permit the marines to stay in Lebanon for 18 months do not provide for expansion of the marine mandate.

Already the marines' support of the Lebanese Army vastly exceeds any normal definition of a ''defensive'' mission, and should a cease-fire prove elusive or break down under the weight of Lebanese politics, the US may have to choose whether to expand it again.