For a full measure of the hazards, not to say the hopelessness, of the U.S. "peacekeeping" mission in Lebanon on its present scale, you have to go back a ways--some would say to the first of a dozen foreign occupations in 1900 B.C. But A.D. 1943 is back far enough.

That's when the French gave modern Lebanon its independence. Ever since, Lebanon has been ruled, when it has been ruled at all, under an unwritten, gentleman's agreement called the National Pact. Its terms, in turn, rested on nothing more substantial than a census taken in 1932, showing a slim Christian majority over the Moslems.

By the numbers, power was forever to be parceled out to a Maronite parliament. There was a little something for each and every Christian or Moslem splinter sect.

The system cracked in 1958 under the weight of a swelling Moslem population. There was a nasty civil war. But the landing of more than 10,000 U.S. Marines and Army troops provided the shock treatment for some neat American diplomacy, reconstituting the National Pact on the basis of the same old and outdated 1932 census.

But the system finally fell apart in 1975, when fiction could no longer be squared with the self-evident fact of a Moslem majority swollen by several hundred thousand Palestinian refugees. The PLO militia forces piled in, after being driven out of Jordan. A much bigger civil war ensued. The entry of a Syrian "peacekeeping" force only made it bloodier. Last year's Israeli invasion compounded the carnage.

The obvious priority now is the removal of the outside occupiers. But the notion that this would restore tranquillity to Lebanon is being daily demonstrated to be no more than an idle, if not disingenuous, White House suggestion. The same may be said for the administration's claims that there would be no need for the U.S., French, Italian and British peacekeepers if the Syrians (and the Soviets) would simply stop sabotaging the withdrawal process.

Serious students of the Lebanese scene know better. They know that the White House is wasting its breath with pleas "to all elements to end this senseless violence and unite behind the Lebanese government to restore national harmony." As far back as 1977, a State Department expert added up the "elements" in Lebanon's shattered society; he was able to identify three separate armies, two police forces, 22 different militias, and 42 political parties. The fragmentation is greater today.

So to believe that today's Lebanon is governable on the basis of a 40-year-old handshake is to believe that President Amin Gemayel is the freely elected, representative leader of Lebanon that the administration pictures, that there is a government with popular support, and that there will soon be a national army answerable to its authority and capable of enforcing its writ once the occupiers go home.

On this last count, you can get assorted estimates of how long it would take to muster and train a genuine Lebanese army with the will and skill to control the sectarian strife. But the most optimistic estimate runs to two years, and some competent observers talk in terms of 10.

So when congressional leaders demand the invocation of the War Powers Act by way of giving Congress authority to limit--or eliminate --the deployment of the U.S. Marines, it is important to recognize what that would mean. There may be a case for cutting losses. If Ronald Reagan can consign Chad to a French "sphere of influence" beyond America's principal concern, Congress supposedly could let the French, British and Italians, by reason of tradition or geography, take primary responsibility for Lebanon.

But that is not what I hear the congressional critics saying. And it is really not the Ronald Reagan line. He is saying that the territorial integrity of Lebanon is critical to our national security.

And yet, in his refusal to confront Congress squarely with the reality of his peacekeeping commitment, the president is flinching in a way that makes the effort all the less sobering to those "elements" he is appealing to for "harmony."

There is an alternative, admittedly painful. It would probably require more international military muscle. But even an expanded role for an enlarged international peacekeeping force is a gamble not worth taking if it is not accompanied by more diplomatic muscle. Lebanon will not find peace until a serious effort is made to deal with the gross inequities--and consequent fierce antagonism--that was built into the Lebanese society as a matter of expediency by a colonial power 40 years ago. president is flinching in a way that makes the effort all the less sobering to those "elements" he is appealing to for "harmony."

There is an alternative, admittedly painful. It would probably require more international military muscle. But even an expanded role for an enlarged international peacekeeping force is a gamble not worth taking if it is not accompanied by more diplomatic muscle. Lebanon will not find peace until a serious effort is made to deal with the gross inequities--and consequent fierce antagonism--that was built into the Lebanese society as a matter of expediency by a colonial power 40 years ago.

Several words were inadvertently dropped from a sentence in yesterday's op-ed column by Philip Geyelin. It should have read: "By the numbers, power was forever to be parceled out to a Maronite Christian president, a Sunni Moslem prime minister, a Shiite Moslem leader of parliament."