President Reagan's Middle East peace proposals have collided head on with the full range of sectarian hatreds and bitterness that have torn Lebanon asunder over the past decade.
Dashed for the time being are hopes that peace in Lebanon might have set the stage for wider US-Arab-Israeli talks, centering on the formation of some kind of Palestinian entity on the West Bank, linked to Jordan.
Now the urgent need is to prevent events of the past two days from plunging Lebanon into the kind of chaos that might bring on a broader Arab-Israeli war.
This process will test US diplomacy to the utmost, for the general Arab perception - shared by many non-Arabs - is that President Reagan, despite tough talk toward Israel, is unable to control the Jewish state.
Focus of the latest crisis is the massacre by Christian gunmen of hundreds of Palestinian men, women, and children in refugee camps of west Beirut. This followed the murder of Lebanese President-elect Bashir Gemayel, leader of Lebanon's Maronite Christians.
Before the Gemayel assassination, some hope had existed that the Maronite leader might create a strong central Lebanese authority, leading to the withdrawal of Syrian and Israeli forces from battered Lebanon.
That hope now is shattered. The camp massacres and Gemayel murder have brought Lebanese sectarian passions - and Arab-Israeli tensions generally - to the boil.
President Reagan and his top advisors, including the secretaries of state and defense, met Sunday to discuss policy moves, including the possibility of returning a contingent of US Marines to Lebanon as part of a peacekeeping force.
Results of the conference were not made public by press time, beyond a statement that ''interagency meetings'' would follow.
The US, France, and Italy, meanwhile, urged the United Nations secretary general to send observers to the sites ''of the greatest human suffering and losses in and around (Beirut).''
France and Italy also indicated their willingness to return their own peacekeeping troops to Lebanon, if the US would do the same. The three powers had made up an international force to oversee the peaceful evacuation of PLO and Syrian troops from Beirut. The Western soldiers have since been withdrawn.
In a related move, UN Secretary General Javier Perez de Cuellar announced that arrangements had been made between Lebanon and Israel for Lebanese Army forces to take up positions in three Palestinian refugee camps around Beirut, including the two where the massacres occurred.
Arab opinion and, to some extent, that of the outside world holds Israel at least partly responsible for the mass murders, because the Israeli Army was in control of west Beirut at the time the killings took place.
The assumption now being voiced, which the Israeli government denies, is that the Israeli Army could have prevented the massacres, because the Christian militiamen gained access to the refugee camps through Israeli lines.
The whole situation subjects US-Israeli relations to enormous strain and poses an excrutiatingly difficult problem for President Reagan.
Earlier, he had condemned the movement of Israeli troops into west Beirut, calling it a violation of the agreement worked out by US special envoy Philip C. Habib, whereby more than 7,000 PLO and Syrian troops had left Beirut.
Now, in the wake of the mass killings, Reagan - expressing ''outrage and revulsion'' - recalled Israel's claim that ''its moves (into west Beirut) would prevent the kind of tragedy which has now occurred.''
The president's statement included a ''demand'' that Israeli troops withdraw ''immediately'' from west Beirut to positions occupied on Sept. 14, the day Gemayel was killed.
''During the negotiations leading to the PLO withdrawal from Beirut,'' said the Reagan statement, ''we were assured that the Israeli forces would not enter west Beirut. We also understood that following withdrawal, Lebanese Army units would establish control over the city.
''They were thwarted in this effort by the Israeli occupation which took place (Sept. 15).
''We strongly opposed Israel's move into west Beirut following the assassination of President-elect Bashir Gemayel, both because we believed it wrong in principle and for fear that it would provoke further fighting.''
Now, if Reagan's credibility as a peacemaker is to survive, he needs to convince Arab leaders that he does not secretly condone Israeli moves and that Israel cannot lightly flout a ''demand'' by the president of the US.
Israel, meanwhile, disclaims all responsibility for the mass killings.
On the contrary, said an Israeli foreign ministry statement in Jerusalem, Israeli troops, when they learned what was happening, intervened to prevent more killing.
It was ''only thanks to the intervention of Israeli forces, which were in west Beirut,'' the statement said, ''that the number of casualties was not much higher.''
''Israel condemns the massacre,'' the government said, and ''will to the best of its power try to prevent the recurrence of such acts between Palestinians and Lebanese.''
Israeli officials insist their army moved into west Beirut to prevent disorder in the wake of Gemayel's assassination.
The Lebanese Army, according to the Israelis, was not filling the vacuum in west Beirut.
It appears clear the Christian militiamen were allowed to pass through Israeli Army lines into the Shatila and Sabra refugee camps where the massacres took place.
Reports from Beirut indicate that the gunmen may have belonged to two right-wing Christian organizations - the Phalangist force which Gemayel had commanded, and the small ''army'' of Maj. Saad Haddad, a breakaway Lebanese Army officer whose forces are armed and trained by Israel.