With the excision of Yasser Arafat and his guerrillas, it will be one down in Lebanon and two to go: the Syrians and the Israelis.

On this much, the Israelis profess to be in full agreement with the United States, whatever their differences over the rightness of the whole enterprise. You can hear no end of giddy talk by Israeli officials and their American apologists about the golden opportunities that now await us: a tranquil "Lebanon for the Lebanese"; a clearer path toward a resolution of the Palestinian problem that has always been at the heart of the Arab-Israeli conflict.

But in its own way -- less bloody but no less confounded by conflicts of interest -- the two-to-go may be the hardest part of the process, and an impediment to the rest. To see why, consider the following exchange with a recent prominent Israeli government visitor to Washington.

With the departure of the PLO, he was arguing, there will be a departure of the remaining foreign forces, and Lebanon will have a "stable" government. And when will the Israeli forces be leaving? Answer: "We will not leave Lebanon until the Syrians leave." And how will that be brought about? Answer: "We hope we will get an agreement, or at least an understanding with the Syrians."

And if not? Answer: "We will stay there." Will the Israelis try to drive out the Syrians, as they have tried to drive out the PLO by force of arms? Answer: "We will not engage in any warfare with the Syrians if they don't engage us."

So there you have it: the independence of Lebanon is to be left to the mercy of the level of trust and general good will between two nations with daggers drawn, and still driven by much the same interests that brought them into Lebanon. The Syrians came to defend the Christians against the PLO in 1976. The Israelis came in to crush the PLO.

In both cases a larger purpose was to guarantee that no force in Lebanon would become predominant in ways that might threaten Lebanon's neighbors. It's this that will make both Israel and Syria reluctant to leave quickly. For what they would then leave behind, even without the PLO, would be a seething caldron of feuds and factions, sectarian hatreds, armed militias, rival warlords -- and no government or army able to maintain law and order.

It is not Lebanon's fault that it is essentially ungovernable when left to its own devices. By careful French design, it was brought into this world severely handicapped. When time came for independence from French mandate in 1943, an unwritten national pact decreed that the president should always be a Maronite Christian, the prime minister a Sunni Moslem, the leader of parliament a Shiite Moslem. The rest of the spoils were similarly divvied up along sectarian lines. The basis of this even balance was a French census taken in 1932.

The wonder is that this system survived through a relatively bloodless civil war in 1958 and only came asunder in a big way in 1975. Even by that time, the indigenous and divided Moslems probably outnumbered the equally fragmented Christians, not counting a massive influx of Palestinian refugees now numbering somewhere in the neighborhood of 400,000. But succeeding Maronite Christian presidents have seen to it that the 1932 census has not been updated.

Neither, in any serious way, has the national pact, which is even less relevant now than it was in 1975. That's when civil war erupted--without benefit of PLO instigation. There was internal anarchy for six months before PLO forces got caught up in the fighting.

It is this sad history that argues against an easy return to a stable Lebanon. Even with all the outsiders gone, what will remain behind is a congenitally unstable society. A prodigious diplomatic effort will be necessary to put together even the appearance of central authority. There are plans for presidential elections this fall. There's talk of somehow disarming the free- lance militia groups, and the rebuilding of a credible Lebanese army.

All this will take time. And time works in favor of the Israelis' and the Syrians' playing it safe. That means settling in: the Syrians in the Bekaa Valley, the Israelis in the south, where they are already taking on the look of an occupation army, with civilian administrators and a stated intention to take a hand on the side of the Maronite Christians in Lebanon's internal politics.

What may begin as a standoff may grow into an indefinite de facto partition of Lebanon. The Israelis would welcome a Christian mini-state as a buffer on their northern frontier. Both sides could blame the other for not going home.

This might introduce a certain stability. But it would not qualify as "Lebanon for the Lebanese." Still less would Israeli occupation of still more "Arab territory" improve prospects for the sort of progress on the wider Palestinian problem that is often offered as the logical consequence of the dismembering of the PLO in West Beirut.