AS ONE STARES at the debris of the dozens fo cease-fires that have been proclaimed and then broken in Lebanon in the last three years, it is idle to say that one of them has more value than the others. Yet the one that the United Nations Security Council demanded and the Christians and Syrians accepted over the weekend is precious indeed. It halted at least temporarily a frightful phase in the calamity that has become Lebanon's permanent way of life. And it forestalled an Israeli intervention in behalf of the Christians, with all the damage to the Arab-Israeli peace effort that could have done.

Through the summer, the militias guarding the Christian Maronite community in Beirut found themselves heavily engaged with the Syrian army, which has been occupying parts of Lebanon for two years, ostensibly to keep the peace in the Lebanese civil war. Did the militias provoke the attack?Which comes first is, by this time, a chicken-and-egg question in Lebanon. In any event, Syria took the occasion, or so it seems, not only to punish the militias but also to make Christian East Beirut uninhabitable and thus to threaten, by demoralization as much as by death, the whole Christian Maronite community. Civilians and civilian factilities were major targets, and Syrian fire was deadly.

Can the weekend cease-fire hold? There is a long chance that it could. One party - the Christian militias - may be frightened and headstrong and resistant to international discipline in the Lebanese manner. But the other - Syria - is something else. Though the U.N. appeal is an implied rebuke to Damascus, the Syrians may well wish to have others take over some further part of the thankless job of keeping the peace in Lebanon. They are probably less interested in provoking the collapse of the Arab-Israeli peace effort than in leaving open an eventual place in it for themselves. They are poorly placed to challenge, at the same time, the United States, which provides aid and a certain political support; and the Soviet Union, their military patron; and France, a traditional friend. All supported the cease-fire call.

Why the Russians? Jimmy Carter personally approached Leonid Brezhnev. Evidently the Kremlin did not want to cause trouble - trouble that could easily get out of hand in a way that could seriously discomfit Moscow - at an otherwise quite propitious moment in Soviet-American relations.

In remains a formidable task to use the time gained by the cease-fire to make a credible start on healing Lebanon's wounds. The Camp David agreements, offering a potential way to lance Lebanon's Palestinian boil, are one possibility. Jimmy Carter's proposal for an international conference to recalibrate Lebanon's Moslem-Christian political balance is another. The matter should be at the top of the international agenda. More people have lost their lives in Lebanon in three years than in all the Arab-Israeli wars combined. The slaughter cannot be allowed to resume.