If you were paying attention to news accounts of the Lebanese war, you probably got the impression (a) that the crunch at the end involved the evacuation of something like 8,000 Palestine Liberation Organization guerrillas holed up in West Beirut; (b) that their departure wiped out the PLO as a political as well as a military force; and (c) that this cleared the way for a mutual withdrawal of Syrian and Israeli forces.

So why was Israeli Foreign Minister Yitzhak Shamir in town last week, arguing vigorously with U.S. officials for the need to remove thousands more PLO fighters before Israel could agree to a mutual withdrawal of Syrian and Israeli forces? And what of Shamir's proposal for a Lebanese/Israeli "security agreement" that might keep Israeli forces in southern Lebanon indefinitely?

A long talk with the Israeli ambassador to Washington, Moshe Arens, is illuminating on both counts. Arens sounds as if he means it when he says that the Israelis want to get out as soon as possible--subject to coming up with "arrangements that will assure our security interests which were the original purpose of the exercise." But the "interests" he enumerates, and the "arrangements" he envisages present enormous impediments.

Impediment No. 1: An Israeli reappraisal of the original strength of the PLO fighting forces in Lebanon. It comes out with the astonishing conclusion that the number of "terrorists" still in Lebanon (8,000-plus) is roughly equal to the number that most people thought had been shipped out and dispersed. Even that number has climbed dramatically, according to Arens, to 15,000 by actual Israeli count of the deportees as they boarded ships.

Even; so, the Israelis insist that 2,000 PLO fighters were overlooked in West Beirut. Arens calculates there may be as many as 6,000 more in the Bekaa Valley, and another 2,000 or so up north in Tripoli. So we are back to Go, with one critical difference: the PLO forces still at large, unlike the forces in West Beirut, are not encircled by Israeli troops.

Instead, the active enforcer of the diplomatic efforts now is the Lebanese army, which has not been able to maintain law and order in Lebanon for the last 10 years.

To quarrel with this new math in Lebanon is to miss the point, which is that the Israelis assert it as a fact. They are not saying "mission accomplished"; they are declaring it unaccomplished. That's why they are declaring with equal insistence that they will not consider a mutual Syrian-Israeli withdrawal (which many people think would be relatively easy to work out) until the remaining PLO forces have been neutralized.

Impediment No. 2: An Israeli-Lebanese peace treaty. The Israelis don't state this as a condition for total Israeli withdrawal. But they effectively discount almost every alternative, including the one Israel first proposed: a permanent, multinational peacekeeping force to police a 25- mile-deep buffer zone in southern Lebanon. Arens notes there was trouble enough winning American approval of the U.S. Marines as a static stabilizer in Beirut. He dismisses United Nations peacekeepers out of hand.

The Israeli "preference" is for a security agreement directly with the Lebanese government. While this might take the form of joint Lebanese/Israeli responsibility for a southern Lebanon "security zone," Arens makes clear that the real Israeli objective is a quick peace treaty with its northern neighbor, to match the one with Egypt to the south. That's what he's been pressing the State Department to urge upon Lebanese President Amin Gemayel. Says Arens: "Gemayel has got to tell his compatriots that we have got to sign a peace treaty with Israel, and do it fast, or this whole thing is going to blow up again."

To recognize this obstacle for what it is, you have only to recall the extraordinary pressure the Israelis were putting on Gemayel's brother, Bashir, only a few dayassassination to make peace with Israel -- and to recall, as well, Bashir's arguments in response. He thought it would doom the kind of "national reconciliation" needed between Christians and Moslems in Lebanon to produce a strong central government. He told U.S. officials that a separate peace with Israel would also wreck his relations with the rest of the Arab world.

President Reagan will have an opportunity today, when he meets with Gemayel, to explore how far Lebanon is now prepared to go in making peace with Israel. But it is hard to imagine why Amin would not see the same pitfalls that his brother saw.

The logic of all this turns on itself. Recent history is all the evidence you need that Lebanon will be hard put to create the sort of stability that would enable it (1) to remove the PLO remnants and (2) to make peace with Israel -- at the same time. What sounds, then, like a case for a speedy Israeli withdrawal becomes a case for Israeli occupation of southern Lebanon indefinitely -- if only because the timetable for Lebanon's political reconstruction is, by definition, indefinite.