First make a cease-fire stick. Then reconstitute the Lebanese government in a way that reconciles a multiplicity of sectarian blood feuds, while also satisfying Syrian interests in more power for the Lebanese Moslems and the Israeli interest in a Christian- dominated regime. Get the Soviets to consent to a United Nations observer force to replace the U.S. Marines (and the rest of the international peacekeepers)-- just as Dwight Eisenhower did in the Lebanese crisis of 1958.

The Syrians, the Israelis, the Iranians and the combat units of the Palestine Liberation Organization would then have no further excuse for hanging around. And a liberated Lebanon, secure and sovereign in its own territory, would live happily ever after.

When serious people are seriously projecting that sort of ultimate solution to the deadly complex of confrontations in Lebanon, you know beyond doubt the degree to which the situation, as they used to say in the U.S. Marine Corps, is FUBAR. Gently translated, that means Fouled Up Beyond All Recognition.

And that's perhaps the most dangerous part of it: not even the policy-makers seem to recognize how fouled up the situation is. Thus they do not recognize the climb-down from current U.S. policy objectives that may be necessary to disengage American forces from Lebanon.

Witness, by way of illustration, the flat White House denial I got the other day of a report that the Reagan administration was getting ready to cut its losses drastically in Lebanon by tacit acceptance of Lebanon's dismemberment: the Syrians and the Israelis would stay pretty much in place and a reorganized Lebanese government would preside over something on the order of Liechtenstein --Beirut and the immediate environs.

Now that is an entirely realistic prediction of how things may well turn out. But it is no more than that, according to my informant. The White House acknowledges no such "plan."

On the contrary, the plan has been to use covering fire from U.S. warships and the threat of escalating U.S. sea or air power to back up a redoubled diplomatic effort to bring about a cease-fire. The theory has been that the Syrians would do a deal rather than risk tangling with the United States or, quite conceivably, the Israelis.

The face-saver for Syria would be a reshuffling of the present caretaker government under President Amin Gemayel, whose election was at the hands of a parliament voted into office a decade ago. Meanwhile, a strengthened army would be demonstrating its ability to enforce the authority of a much more representative government. There would be no internal sectarian power struggling for outsiders to exploit. But the hardest part would remain: persuading the Syrians, with their historical and regularly reiterated claims on Lebanon, to match Israel's agreement to withdraw.

To understand why this is the hardest part, it is helpful to examine the U.S. experience in Lebanon in 1958--not as a model but for the fundamental differences in the situation, then and now, that make the current propects look so dismal.

Then, Syria was not, strictly speaking, Syria. Nor was it snuggled up to the Soviet Union. It had linked itself with Egypt in a so- called United Arab Republic, precisely because dominant Syrian military elements had grown fearful of over-dependence on Moscow. Then, Lebanon was caught up in what could rightly be called a "civil war." Moslem rebels had taken up arms against a Christian Maronite president who was maneuvering to stay in office beyond his prescribed term.

True, the Syrians and Egyptians were operating a clandestine supply line to the Moslem rebels. But they were never proven to have been in cahoots with the Soviets-- though that was the pretext for landing some 14,300 Marines and Army Airborne troops. There was not even a whiff of hostilities involving U.S. forces. It took U.S. Special Envoy Robert Murphy only 10 days to work out a political solution.

With order thus restored, the Soviet Union had no grounds for opposing a United Nations force to replace the Marines--the more so since such a force was already on the scene, having arrived at least a month before the U.S. intervention.

The lesson from Lebanon, circa 1958, is simply stated: if you have a political settlement, you don't need the United Nations to extricate American troops--they were already on their way out of Lebanon in 1958 before the United Nations passed its peacekeeping resolution; if you don't have order, the United Nations cannot restore it. That job must remain the responsibility of the diplomats and the peacekeeping forces--as it was in 1958.

The awful difference is that the job now is immeasurably more dangerous and complex, and the reach of U.S. policy is so far beyond what would seem to be its grasp.