Beirut — From the hills in the evenings, the difference between east and west Beirut is like night and day: east Beirut glitters with light, west Beirut is dark.
Israel's month-old war against the Palestine Liberation Organization had changed July 5 from a lightning military campaign to an ugly siege - a siege on which all sides were putting a brave face. But the fact remained that the PLO was trapped, Israel was bogged down, and Lebanon was even less in control of its destiny than ever before.
Israeli and Phalangist Lebanese had cut almost all traffic entering west Beirut. They had also - at least for the night of July 4 cut off water, electricity, and phone service to that side of the city. The Israelis also increased the military pressure on west Beirut by stepping up shelling around the airport and Borj el Baragne refugee camp.
Israeli tanks July 5 began assembling in strength near crossing points into west Beirut and artillery fire became intense. (Lebanese President Elias Sarkis was injured Monday when shells landed outside the presidential palace in Baabda, southeast of Beirut, according to Beirut Radio. The radio report said the shells landed in the palace yard, but it was not known who fired them.)
Meanwhile by 4 p.m. (local time) a new cease-fire had been declared. This repeated the month-old pattern of flare-up/cease-fire/flare-up/cease-fire that has characterized this conflict.
''The basic situation has not changed,'' Israeli Col. Paul Kader told correspondents July 5 as artillery sounded around his office in Baabda.
''We still hope for a political solution. The problem is that if you accept things as they are, that creates a new status quo with the PLO in west Beirut, us in the east, and everything bogged down. . . . It does look as though the status quo is taking away the sense of urgency (to the negotiations).''
The shelling and the sealing off of west Beirut, Colonel Kader indicated, were meant to upset the status quo. But in west Beirut, Lebanese Prime Minister Chafik Wazzan warned that if Israel did not loosen its hold on the city, the negotiations would be called off. George Habash of the Marxist-oriented Popular Front for the Liberation of Palestine said the PLO would not negotiate with the Lebanese government unless the Israeli siege was reduced.
And in Israel, Prime Minister Menachem Begin seemed to dash any diplomatic hopes that a limited PLO presence could be left in Lebanon - a proposal that gained currency during the past weekend - when he said, ''We shall not tolerate even one terrorist in Lebanon.''
Diplomacy seemed to be shot because, in a typically Middle Eastern way, all parties could rationalize all possible outcomes.
True, it is costly for Israel to maintain an estimated 90,000 troops in Lebanon. But military analysts say that Israeli casualties in an assault on west Beirut would exceed 700 dead - a number that could well cause dissent in Israel to bring down the Begin government. Moreover, Israel could, while maintaining the siege, consolidate its hold on southern Lebanon, building a security network from Sidon south, perhaps tapping the rich water resource of the Litani River, and arresting PLO members out of the limelight.
The Israeli siege keeps PLO forces united in the face of a nearby enemy. But if the Israelis stormed west Beirut, the PLO also could benefit, despite heavy loss of life, by causing the world to blame Israel for one of the most destructive urban battles since Stalingrad. Moreover, the PLO would have an ''Alamo'' to which to rally support in the future.
While the Israeli siege is going on, the right-wing Phalangist forces can spread their power throughout the Israeli-occupied territory without actually having to fight for or defend their holdings alone. And yet, Israeli elimination of the PLO would make the Phalange the strongest force besides Syria and Israel, in Lebanon.
Syria, meanwhile, can tighten its de facto governance of the Lebanese Bekaa Valley and the area around the northern coastal port of Tripoli. Yet Israeli destruction of the PLO in Beirut would eliminate independent Palestinian operations, leaving Damascus-controlled Palestinians as the strongest, most radical Palestinian force.
The biggest loser in a prolonged siege would seem to be the Lebanese government. Israel, Syria, the PLO, and the Phalange are all stronger than the Lebanese central government. The government has been unable to act even to deploy the Lebanese Army into west Beirut.
But if and when the final Israel-PLO clash begins - or if and when the PLO leaves Lebanon - Lebanese government leaders will probably see what little power they now hold, by virtue of being middlemen in peace negotiations, vanish before the driving force of an Israeli-backed Phalangist government under the Phalange's dynamic young leader, Bashir Gemayel.
The diplomats' problem, therefore, has been that no party has seen enough benefit in any particular peace proposal. A fight to the finish seems as good as the status quo.
Which takes the story full circle back to the Israeli siege. A tightening siege, according to Israeli officers at Baabda, may help speed up the negotiations, or it can be a prelude to a heavy Israeli attack. Or, in the words of one officer, ''It can underscore the point that we mean business.''