The 1,200 U.S. Marines in Lebanon could remain there for a minimum of four to six months, according to government officials who say Americans should be braced for further casualties such as the death of one Marine and wounding of three others last week when an artillery shell exploded during mine-clearing operations.

President Reagan has said the Marines must stay as part of a multinational force until reestablishment of a Lebanese government with real authority and removal of Israeli and Syrian troops from the country. By all assessments, these are the prerequisites for pushing the president's stalled Middle East peace initiative beyond the talking stage.

Reagan and other U.S. officials have stressed repeatedly that the Marines' mission is meant to be a short one. They used phrases such as "of limited duration" and "a few weeks."

Behind the scenes, however, officials speak about a minimum stay of four to six months--for the Lebanese government and army to reestablish authority in Beirut and then for that authority to spread in stages into areas vacated by withdrawing Israelis and Syrians. As more than one policy-maker noted, little seems to go as scheduled in Lebanon.

This would not preclude an agreement being reached more quickly on the framework for withdrawal of foreign forces. U.S. special envoy Philip C. Habib spoke in terms of a few weeks during a visit to Cairo late last week, according to Egyptian officials, and Israeli Foreign Minister Yitzhak Shamir said yesterday that Habib has told Israel that Syria is "ready in principle" to withdraw.

Such progress on withdrawal, U.S. officials say, may be enough to give the "considerable ferment" in the Arab world an opportunity to coalesce on the Reagan peace initiative. A meeting between Jordan's King Hussein and Yasser Arafat of the Palestine Liberation Organization is said to be imminent, and Saudi Arabian Foreign Minister Prince Saud has canceled a scheduled trip to the United Nations, possibly to be on call for the Hussein-Arafat talks.

The scenario, as charted by administration officials and longtime observers of U.S. Middle East policy, is not particularly complicated on paper.

Syria and Israel have legitimate security concerns in the region -- Israel about its northern border and Syria about the fact that Israeli troops are poised along two Syrian borders.

Each may have said in principle that it does not want to stay in Lebanon, but neither is likely to withdraw until it believes its security needs are met.

Each traditionally has sought a government in Beirut that leaned in its direction. Syria long has felt that Lebanon is within its sphere of interests, and Beirut has been the traditional port of entry for Damascus. Israel has sought to minimize such Syrian influence by joining forces with the Lebanese Christians and, more recently, by seeking a formal treaty guarantee.

The key to possible success, in this analysis, is establishment of a truly independent Lebanese government. Syria, weakened after a military drubbing by the Israelis and by continuing internal dissent, is viewed as vulnerable to pressure for a pullout. Israel, staggered by the reaction to the Beirut massacre, is pictured as open to accepting something less than a treaty guarantee, at least in the short run.

"Up to the massacre, it was impossible. No Israeli government would accept Lebanon's word on the security of its northern border. There was some faith in Bashir assassinated president-elect Bashir Gemayel , but they still wanted that piece of paper," one observer noted.

"Israel has demanded more than an independent Lebanon as security. That is why Maj. Saad Haddad's [Lebanese Christian] force is there and is being expanded. As a result of the massacres, however, there may well be less support for a direct or indirect presence."

French Foreign Minister Claude Cheysson spoke in New York last week of the need for guarantees for Israel against any threat across its northern border, probably accompanied by an increased international troop presence in southern Lebanon.

Israel's distrust of the U.N. force currently in the area has been emphasized recently, so it remains unclear if an augmentation of that force could prove acceptable.

New Lebanese President Amin Gemayel, Bashir's brother, has not ruled out a treaty with Israel but says parliament must decide the issue.

The tactical arrangement that special envoy Morris Draper is believed to be seeking is a phased withdrawal under which Lebanese Army units would move into areas vacated by Israelis, Syrians and Palestinians.

"As the Lebanese Army can expand, Israel and Syria would pull back. But that always leaves the chance for fringe elements to do something to show the Lebanese Army doesn't 'have control,' " one source noted.

This brings the issue back to the first question: establishment of control by the Lebanese government, essentially an element of trust in a society sharply divided along religious lines and well aware of past atrocities.

U.S. policy-makers have viewed as positive signs the decision of the Moslem leadership to support Amin Gemayel to replace Bashir and the president's visit to predominantly Moslem West Beirut last Thursday as a unity gesture. Leading Moslem figures in Beirut also have been blaming Israel, not the Christian militia close to Gemayel, for the recent massacre in two Palestinian refugee camps, one U.S. analyst noted.

"They all know who did it, but it is their way of saying they are giving Gemayel another chance to prove that he has control over his crazies. That is a good sign," this analyst said.

"The central problem is trying to assert central government control over the fringe elements. Somehow, these elements will have to be purged or suppressed," another observer said.

Accompanied by military experts, Draper is said to be surveying the needs of the Lebanese Army and prospects for its gradual deployment. This process is expected to be followed by an aid package for the Lebanese military and serious talks with Israel and Syria on a pullout.

Officials say it is unrealistic to begin hard bargaining on the pullout until establishment of a firm date for the Lebanese Army's effective organization and deployment. For an army swatted aside by outside forces and internal militia for years, this is no small feat.

Yet, there appears to be greater optimism on the army's future than on the continued tacit agreement of Lebanon's political leaders to allow emergence of a strong central government.

Habib won the trust of Palestinians and Lebanese Moslems when he negotiated the PLO withdrawal from Beirut. Whether he or Draper can do that again remains to be seen, because of mistrust generated by the massacre.

According to well-informed officials, Habib is said to have received assurances from Israel and Lebanese Phalangists that the PLO camps would not be touched after the forces withdrew. He passed these assurances to Arafat and leading Lebanese Moslems involved in the talks.

That is what Arafat and Lebanese Moslems are believed to be citing when they talk of broken U.S. guarantees for the camps. The breaking of these assurances is said to have been a major factor in Reagan's anger at the Israeli government.

Ability to reestablish that element of trust between U.S. negotiators and leading Lebanese politicians is now seen as a decisive factor in how quickly the Marines can be withdrawn.