It sounds simple the way the Israelis put it: everybody out of Lebanon, except the Lebanese. Lebanon will then revert to the good old days before the Syrians moved in and the Palestine Liberation Organization planted a headquarters and a considerable military establishment within artillery range of Israel.

The United States, for its part, will have a made-in-Israel opportunity to consolidate its Mideast influence at the expense of the Soviets, whose Syrian surrogates will have been humbled and whose PLO prot,eg,es will have been either crushed or scattered. Presto! The Arab-Israeli conflict will be riper than ever for one, big, happy comprehensive settlement, with the "moderates" among the Arab states and some silent majority of pliable Palestinians all eager to pitch in.

Here's how a "senior Israeli official" put the proposition to Washington Post correspondent William Claiborne in Jerusalem: "The United States has been handed by us, on a silver platter, political and strategic advantages it could never have dreamed of before this operation. We hope the Americans recognize the opportunity and don't let it slip away."

Well, thanks a lot. But that's not exactly what the Israelis first told us was the original purpose of the exercise. For the idea of clearing a 25-mile buffer zone in Southern Lebanon (and putting northern Israel out of reach of PLO artillery and rocket fire), a reasonable case could be made.

The Reagan administration was thus encouraged to move from opposition to any invasion to a routine request for a cease-fire and a unilateral Israeli withdrawal. It is less easy to explain the feeble protests against the widening carnage, and the ultimate acquiescence to no withdrawal by the Israelis unless the Syrians and the PLO also go away.

The effect of that easy dissolution of the original American position is enough to raise a large question about the supposedly great opportunity that the Israelis now say is there for the seizing by the United States. For its effect is to suggest strongly that the United States was either (a) incapable of influencing Israel or (b) didn't want to. Secretary of State Alexander Haig insists fervently that no "collusion" was involved.

It doesn't really matter. The appearance of the Israelis using American-supplied weapons, in a manner that at least raises legal questions--without any effective application of American restraint--is what will matter to those "moderate" Arabs who would have a rather important role to play in the fulfillment of the peace prospects the Israelis say are now so close at hand.

"Nobody in the Arab world is going to believe that the Americans didn't have a hand in it," says an Arab diplomat. If so, it is not immediately apparent how that suspicion translates into greater American influence in promoting an Arab-Israeli settlement. And that, perhaps is only the least of what's wrong with the new line put forth by Israel and its American supporters in justification for what has now developed into an effort to destroy all vestiges of the PLO in Lebanon-- including its Beirut high command.

A case can be made that, deep down, the more moderate Arab leaders may welcome the destruction of an extremist element. But up-front, they can't say it--or even act as if they thought it. And even deep down, they will have to be wondering what methods the PLO remnants may now resort to.

If the potential for conventional warfare is gone, stepped-up PLO terrorism is rated by most experts here to be the most certain alternative. "That's their only weapon now," says one American authority. This means more violence, in all likelihood, on the occupied West Bank. It probably means more assassination attempts against Israeli officials worldwide. And it may also mean intensified PLO efforts to subvert the moderate Arab regimes.

"The general effect will be to radicalize, not moderate, the Arab world," according to one leading Mideast expert.

For the short haul, Israel may be more secure--at least for as long as Israeli forces occupy southern Lebanon. But the Israeli army is made up of reservists whose normal civilian jobs are vital to the Israeli economy. So over the long haul, time is not necessarily on Israel's side.

The PLO has been wiped out before, only to be revived. Those who talk about "reconstituting" Lebanon tend to forget that before the Syrians were invited in as "peace- keepers," there had been more than one civil war born of Lebanon's own chronic religious, political and social instabilities.

Negotiating the withdrawal of the Syrians; the suppression, somehow, of a PLO presence; the recruiting of an international peace-keeping force; the restoration of a strong central government in Lebanon--all that can't be done quickly, if it can be done at all.