Winding through the center of Beirut is a 200-yard-wide strip of charred and crumbling wasteland known as the ''green line.''
It runs from Beirut's seaport in the north, past the country's parliament and National Museum buildings, and down to the international airport on the south, separating east from west, Israeli and Phalangist from Palestinian and Syrian and leftist.
Only by default is the Lebanese Army in the green line today, and by the weekend parts of three more armies - French, Italian, and American - should be moving into this no man's (or, perhaps, ''everyman's'') land.
It is through this newly neutral zone that two crucial transactions are to take place in the next few days: the exodus of Palestinian guerrillas and the election of a new president for this long-troubled republic.
On Aug. 18, bulldozers were doggedly trying to scrape the dirt from huge earthen barriers erected in the recent Israeli-Palestinian fighting here. Passive-looking Lebanese soldiers picked their way around bomb craters and milled about the front door of the Villa Mansouri, where parliamentarians were to meet Aug. 19 to begin trying to choose a successor to President Elias Sarkis.
Although they will have to crunch through chipped stone and broken glass, it appears the delegates will, indeed, be able to meet on at least technically neutral ground - under Lebanon's cedar-tree flag, hypothetically protected by Lebanon's dubious Army.
An example of how tenuous is the Lebanese Army's position came Aug. 18 when Palestinian forces in the museum area broke the six-day-old ceasefire by firing rocket-grenades. Correspondents watched the Lebanese in the green line quickly duck out of sight. Then Israelis responded with their tank cannons. (The breach of ceasefire seemed of limited duration at this writing.)
This neutral zone will serve the troops of the new multinational peacekeeping force that, if all goes as planned, will begin supervising the departure of Palestinian guerrillas from Lebanon this weekend. At this writing, the plan developed by American diplomat Philip Habib seemed to be maturing properly, and all that remained was formal agreement by all parties concerned. On Aug. 18, Lebanon's Cabinet approved the plan, and on the 19th it was to ask for the multinational force to be sent.
If final agreement by Israel and the PLO is forthcoming, French forces are expected to take up positions in the seaport by Aug. 21. Then American and Italian soldiers are to enter the green line also, internationalizing Beirut from north to south.
Israeli officials in Beirut say they believe the bulk of the PLO will leave, knowing that this is their only chance to clear out. After the multinational force departs (as things now stand that would be 15 to 30 days after it enters the capital), west Beirut will come under the control of the Lebanese Army.
Since the Phalange and the Israeli Army will still be here at that time, it is probable they will enter the west side of the city also in order to hunt down PLO stragglers and to exert dominance over the leftist Muslim militias on that side of town.
''When the PLO leaves, there will still be a lot of shooting in this city,'' an Israeli spokesman predicted Aug. 18. ''I'm sure the PLO will tell people to stay behind, and many of them will pop up later and surrender.''
Lebanese sources say that, if there are no hitches in the evacuation plan, an estimated 10,900 guerrillas will leave over the next two weeks. The first batch of 2,000 would go by sea to Cyprus and then fly to Iraq, Egypt, and Jordan. Then the bulk would go to Syria.
While all this is going on, the Lebanese parliament will be trying to elect a new president - or will be in the midst of a crisis of presidential succession.
Almost the entire Maronite Christian bloc of deputies has endorsed Bashir Gemayel, the leader of the Phalange. The Maronite hierarchy is pushing hard for the country's deputies to convene a quorum as early as Aug. 19, confident that Mr. Gemayel will be elected president if only the parliament can meet.
But Muslim deputies, led by former Prime Minister Saeb Salam, Aug. 18 issued a statement saying they would not attend the assembly until a national entente had been achieved and all foreign forces - in particular the Israelis - had left Lebanon. There were unconfirmed reports that Syrian forces had closed the route into Beirut from the Bekaa Valley and Tripoli, preventing deputies from these areas from attending also. These developments cast grave doubts on whether a quorum could be gathered.