Beirut — Lebanon has entered perhaps its most delicate stage since civil war broke out more than nine years ago. The final withdrawal of French peacekeeping forces Saturday left Beirut without a major foreign armed presence for the first time since 1975. In effect, this provides the first test as to whether warring factions can control themselves militarily in the Lebanese capital.
Many Lebanese were surprised by the relative calm along the city's dividing ''green line'' - broken only by the odd cackle of automatic rifle fire - after the French turned it over to the Internal Security Forces, a local paramilitary body. Forty French observers, armed only with pistols for self-defense, were left behind after the French pullout and the announcement that American and Italian ships would leave Lebanese waters.
Officials from Christian and Muslim factions privately endorsed a comment from Lebanese Foreign Minister Elie Salem: ''Time has come for Lebanese to rely on themselves.''
Yet the doubts of the average Lebanese were reflected by the Beirut newspaper , Middle East Reporter: ''How will Lebanese policemen, retired Army officers, and Army recruits succeed in stabilizing the security situation when . . . 6,000 soldiers from four major Western powers were as ineffectual in checking the Lebanese civil strife as were 30,000 Syrian troops before them? Nor were 75,000 Israeli soldiers who invaded Lebanon in the summer of 1982 able to change conditions.'' Syrian forces remain in eastern Lebanon; Israeli troops still occupy the south of the country.
One major variable working in favor of peace, according to Western military sources, is the fact that all factions are having trouble getting resupplied with ammunition and war material. Syria has reportedly cut back on shipments to leftist Muslim groups as one way of forcing them to adhere to the latest cease-fire.
A spokesman for the Christian Phalangist ''Lebanese Forces'' said they have supply problems because of limited funds and sources of arms. He said Israel had not provided large military shipments since last summer. A Western official said the Phalangists could last no more than five or six days in any major battle.
Another possible factor is the unspoken threat of Syrian troops once again marching into Beirut to divide the warring rivals so that the politicians can talk. Under an agreement reached last month at talks in Lausanne, Switzerland, the rivals have six months to come up with constitutional reforms. The main dispute is over the balance of power between the majority Muslims and ruling minority Christians.
Although diplomats here say that the Syrian leadership does not favor this option, they suggest that President Hafez Assad is prepared to consider it to ensure the success of his Lebanese peace initiative - and preserve his own credibility.
Over the past week, Syrian officials and the state-run media have issued a steady stream of warnings to Lebanese factions. The semi-official newspaper Tishreen wrote: ''Any deviation from the (Lausanne) resolutions will be met by a firm Syrian retaliation.'' Syria ''will not appease anyone in the efforts to achieve the great goals of Lebanon. It will not allow anyone to undermine this victory.''
Syrian Vice-President Abdel Halim Khaddam, who was the chief mediator at Lausanne, told an Arabic magazine that ''the time will come when Syria will find it difficult to permit the fighting and divisions in Lebanon to continue.''
A new committee of opposition and government representatives is in charge of enforcing the latest truce. The group has met daily under President Amin Gemayel since last Wednesday to look for ways to ease tension. The ability of all major groups to continue the talks begun in Lausanne boosted the feeling that there is potential for peace. Joseph Abu Khalil, a senior official of the Phalange Party, predicted that the situation in war-ravaged Beirut would be normalized ''within the next 10 to 12 days.''
Ghassan Siblini of the Shiite Muslim Amal movement more cautiously said: ''Progress has been made on several issues.''
Yet the fact remains that there is more danger than hope in Lebanon, according to militia officials and key diplomats. One major act of sabotage by extremists not represented in the new committee could easily trigger a breakdown. It is also not in the interest of any faction to obey the cease-fire for long, since military action is the prime means of pressuring the government and rivals on political reforms.
Most important is the power-sharing dispute, which was not solved in Lausanne and has not yet been tackled back home. A 32-member group, which is to find a new formula for evening the balance of power, has not been established. Without progress in the near future, Muslim leaders in particular - such as Shiite leader Nabih Berri and Druze leader Walid Jumblatt - are likely to come under pressure from hard-liners to withdraw their participation.