The resignation of Prime Minister Shafik Wazzan and his nine-member Cabinet is the worst crisis to face President Amin Gemayel since his election 17 months ago.
Although the Lebanese President has asked Mr. Wazzan to stay on in a caretaker capacity until a new government can be organized, it seemed clear that a new cabinet with any credibility would be difficult to find in light of general Muslim discontent.
The Cabinet's resignation on Sunday followed three days of fierce fighting between the army and antigovernment militias which accentuated the now almost irreparable divide between Christians and Muslims.
In effect, the latest round in the nine-year conflict forced the long-simmering crisis to a head, as Muslims in the Cabinet and the army indicated opposition to serving for a Christian-dominated government that has so far shown few signs of willingness to share decisionmaking with the majority Muslim community.
There was also significant evidence Sunday that Lebanese army officers and troops of Muslim sects had laid down their arms rather than fight Shiite and Druze militias on behalf of the government.
Several army checkpoints in Muslim-dominated west Beirut were handed over peacefully to Shiite Amal gunmen, including two close to the US Marine base near Beirut's international airport.
But it was not clear whether the officers, who were subsequently seen wandering in Shiite suburbs, had officially defected or had taken a temporary action to help force political negotiations that would even the balance of power.
Prime Minister Wazzan has threatened to resign on several previous occasions. Twice he handed formal letters to President Gemayel, but on each earlier occasion he relented under Lebanese and US pressure.
His commitment to carry through this time reflected the depth of the crisis under the duress of a deadlock in reconciliation efforts and of on-going fighting that has increasingly taken on political overtones.
The premier, a Sunni Muslim, said the resignation ''stemmed from our conviction that all factions should inevitably participate in shouldering the responsiblity in the current grave situation to prevent further deterioration and to work for salvation.''
And in a nationwide broadcast Friday he had appealed: ''Decisive stands should be taken to restore people's peace of mind, the country's stability, and the world's confidence that Lebanon is still alive.''
Mr. Wazzan and the three Muslim members of his Cabinet forced the move after announcing they agreed on ''the necessity for change,'' an indirect slap at Mr. Gemayel, a Maronite Christian, and his supporters. The Maronites have balked at accepting major reforms to end the bloody and bitter Christian-Muslim rivalry.
It appears that the lapse of time has opened the way for new militancy among Muslims that may even threaten Mr. Gemayel's hold on the six-year presidency. Druze leader Walid Jumblatt recently called on Mr. Gemayel to leave office as a condition for peace.
And on Sunday Shiite Amal leader Nabih Berri claimed, ''All the political ways are now closed with President Gemayel.'' Asked if he meant Lebanon needed a new president, he replied: ''I should say, yes.''
Also over the weekend, Mr. Berri issued an appeal for all Muslim ministers to leave the government, and for all Shiite members of the army to lay down their arms rather than fight their own people.
Shiites make up the largest single share of the 37,000-man Lebanese army, which was seen as the major hope for eventually restoring order and sovereignty to the country. But Western military analysts have been predicting for some time that any major confrontation before reconciliation could take place would again split the armed forces on confessional lines, as first happened during the 1975- 76 civil war.
The danger for Lebanon now is that the current round of violence, which has killed at least 70 and injured more than 250 over the past three days, will continue until there is a political solution.
And with each passing day, reconciliation appears more remote. Sunday's events were interpreted by Western diplomats as a major victory for the complex network of opposition forces, leaving Mr. Gemayel boxed into a corner.
Yet with each new condition placed by the opposition, the President and the Christian Phalange Party he once served grow more intransigent rather than more realistic about the terms of peace.
The gap has been most evident on the issue of the May 17 agreement between Lebanon and Israel on the withdrawal of foreign troops. The agreement was mediated by the United States.
Muslim groups have demanded that the pact be scrapped, because it allows a residual Israeli military presence in Lebanon, before they will return to the Geneva reconciliation talks that were suspended last November.
Western officials in Washington and Beirut claim a compromise formula had been worked out to formally ''freeze'' the agreement. But so far the Gemayel government has refused to take even this partial step, instead arguing that political reconciliation cannot take place until all foreign forces are out of Lebanon - through agreements like the May 17 pact.
Muslims, however, fear the Christian minority will then have such a hold on power they will be unwilling to concede reforms. The Muslim response has been to exert leverage through use of arms.
As a Western diplomat noted: ''It's a chicken and egg argument basically, but a very complex one, involving very stubborn sides.''
If President Gemayel is unable to find a new premier and cabinet, there is informed speculation that he may resort to a military council to govern Lebanon. But Western diplomats suggest this option would in effect be admitting total defeat of peaceful political alternatives. They add it could allow the violence to spread into another full-scale civil war.