THE CEASE-FIRE was holding last night in West Beirut, and it appeared that the city's ordeal might be ending. Surviving PLO forces had sought the cover of its half a million or more (unconsulted) residents, counting on Western pressure --in the absence of Israeli scruple--to keep the Israeli army at bay. Israel's strategy for forcing a PLO surrender, however, rested on convincing the PLO and everyone else that the Israeli army would go in.

To his credit, Ronald Reagan finally became alarmed enough by the devastation the Israelis had wrought on the way to Beirut to try to stay their hand. For a while, his counsel made them--and evidently Secretary of State Haig, too--fear that he might neutralize their tactics for intimidating the PLO. The PLO, though, decided not to call the Israeli bluff. Thus was West Beirut reprieved.

It still is not certain that the siege of the city will be permanently lifted. If it is, however, the stage may be set for Lebanon's halting re-emergence as a state in control of its own territory and destiny. Should Israel have adopted the goal, beyond combating terrorism, of removing Palestinian and Syrian forces and "reconstituting" Lebanon? Should it have used such bloody methods in pursuing that goal? Argument on those questions will continue, but it cannot be permitted to get in the way of affording Lebanon whatever benefits it can draw from its latest agony. Stung by the impression they have conveyed of indifference to Arab lives and American interests, the Israelis have a major interest in replacing their army with an authentic Lebanese authority in snap time.

The Palestinians can claim some face and profit in averting a final Israeli onslaught on Beirut and in witnessing a change at the American State Department of at least symbolic satisfaction to them. The reality remains that the PLO has lost its military resources and pretensions and its lone base of operations. As a result, many people now predict, the movement will become increasingly fragmented and radicalized. It could happen. But it should not be allowed to become a self-fulfilling prophecy. This is the worst moment to close off all political vistas to the Palestinian national movement.

It depends, ultimately, on the Israelis. They can take comfort in having broken the PLO's sword. They can take no comfort in having added to its store of hate. Representing, as it still does, Palestinian nationalism, the PLO is no longer a real military threat, if it ever was--though terror is another matter. It remains the force that compels Israel to lead a lonely, dangerous, stranger's life in its region. The dominant Palestinian grievance, Israeli rule over a million-plus Arabs in the West Bank and Gaza, endures. The true service the United States can perform for Israel, as for itself, has not changed: to help Israelis and Palestinians find common ground.