If President Reagan really intends to keep the Marines in Lebanon until a stable government emerges here, he might consider building permanent housing for them. Lebanese stability, like Lebanese unity, is mere wishful thinking.

In his first public act as president of Lebanon, Amin Gemayel sought to suggest otherwise. Visiting the tomb of the unknown soldier in the so-called "Green Line" -- the no- man's land of shell-holed buildings and rubbled streets sepating Moslem west Beirut from the Christian eastern sectors -- he proclaimed the divided city "again the capital of all Lebanon" and declared "a reunification of the hearts of the Lebanon."

In a country torn by civil war, foreign invasions and occupations, violent religious antagonisms, regicide, savage civilian massacres, private blood feuds and destabilizing international intrigues, Gemayel's words no doubt echoed the dreams of a vast majority of his nation's 3 million inhabitants.

But to one who knew this once resplendent Mediterranean city a decade ago (when it boasted, with some justification, of being the "Paris of the Middle East"), and one who has returned here repeatedly to chronicle its vicious slide into chaos and anarchy, the new president's words sounded like more of the rhetoric that has long cloaked the truth: Lebanon, if it ever was a viable political entity, exists today in name only.

Many Lebanese, especially those of Gemayel's Christian rightist Phalange party, like to blame their state's collapse on the machinations of the now-departed Palestine Liberation Organization, which was for so long a state within a state. But the disunity and violence loose in the land are as Lebanese as the cedar trees that give the country its national symbol. And they are just as hardy.

To talk of the "reunification of the hearts" is to gloss over the historical fact that Lebanon's hearts, minds and bodies have never been truly united and probably never will.

Disunity was the case in antiquity, when the Canaanitic people along its coast gave birth to Phoenician mercantilism from a string of competing city-states. It was the case centuries later, when Christianity first took hold there as the Roman Empire declined. It was the case in the 7th century, when the banner of Islam swept out of the Arabian peninsula to force a Christian retreat into the steep, isolated valleys of Mount Lebanon. It remained the case under Ottoman rule, when the first abortive efforts were made to raise the Lebanese above their violent feudal loyalties to family, clan and tribe.

In our century, as Lebanon has passed from Turkish to French to independent rule (in 1943), it has been the explosive contentiousness of its people that has characterized the land. Clan continued to fight clan, village warred against neighboring village, and its 17- odd religions -- divided almost equally between Christian and Moslem sects -- kept alive conflicts that go back to the Crusades. Interconfessional civil war, or such obscenities as the recent Christian massacre of Palestinian refugees in Beirut, are not so much social aberrations as aspects of a dark national norm.

When I first came to Beirut as a foreign correspondent in 1970, these demons were disarmingly quiescent. It was an era which some nostalgically refer to as the "good old days," a time of optimism and prosperity, a time when the city's efficient and secretive banking system, its unfettered communications and its fast lifestyle made it the capital of the whole Middle East.

At first glance, Beirut was both sophisticated and exotic. There were fancy boutiques with Parisian fashions along traffic-clogged Hamra Street, as well as captivating traditional markets in the narrow streets of the Basta. Saudi Arabian sheiks in white robes jostled Western businessmen in gray suits around the gaming tables at the Casino du Liban. In summers the beaches teemed with bikinis, and in winter the ski resorts at the Cedars and Faraya were mobbed.

Academics, diplomats, journalists and spies swapped tales from the comfortable leather armchairs of the St. Georges Hotel bar, and it seemed the evenings were only for fine dining at French restaurants like Chez Temporel or for spirited cocktail parties on balconies of towering apartments overlooking the placid Mediterranean.

First impressions were deceptive. Beneath this patina of sophistication lurked an ugly current of violence. Already in 1970, Lebanon's population was reputed to be one of the most armed in the world. Gun ownership was a sign of manhood, and it seemed no one moved about the city without a gun tucked away in a pocket, a glove compartment or under a car seat.

This reality was brought home shortly after my arrival when I read that a British diplomat had been shot to death in the head by an irate taxi driver. It seemed the diplomat had accidentally brushed the taxi with his car in traffic and continued down the road unaware. The taxi driver slammed on his brakes, pulled a gun from under his seat, and shot the diplomat through his rear window.

The first day I went to my office in the An Nahar newspaper building, I was startled by the arrival of former president Camille Chamoun to visit the newspaper's editor, Ghassan Tueni, until last month Lebanon's urbane ambassador to the United Nations. Chamoun, a Godfather-like pol in white suit and dark glasses, drove up to the building in a convoy carrying his own private army. I learned quickly that no politician of note was without his own well-armed militia.

A month later, Lebanon held presidential elections. The vote in parliament divided equally between Elias Sarkis, then head of the Central Bank, and Suleiman Franjieh, a crusty Christian mountain chieftain from Lebanon's north. With the speaker of parliament holding the deciding vote, Franjieh's son, a parliamentary deputy, moved to the dais and unholstered his pistol before asking the speaker how he intended to vote. The speaker voted for Franjieh, who duly became the fifth president of Lebanon.

Sarkis was elected president in 1976 after Franjieh's term expired in the waning days of the savage, 18-month civil war that finally laid to rest any pretentions the country had to civilization.

Franjieh, I soon learned, became a political force in 1957 by leading a group of his Christian clansmen into a church in a neighboring northern village and, during a requiem mass, murdering half a dozen rivals as they stood at the altar. The president-to-be then fled into exile in Syria until his transgressions were forgiven two years later and he was allowed to return to resume his political career.

Conditioned by their culture to view such institutionalized violence as normal, my Lebanese friends at the time liked to talk of the "miraculous" success of their governing institutions in containing the centrifugal forces of this splintered society. This, they said, was due to the unwritten National Pact of 1943, whereby the country's Christian and Moslem leadership agreed to a complex -- and still unworkable -- formula for sharing power and jobs.

Based on a dubious 1932 census that gave Lebanon's Christians a slight majority over its Moslem population, the pact provided that Christians would henceforth get six government positions -- in the executive, legislature, judiciary, bureaucracy or army -- to each five parceled out to the Moslems. Thus it came to be that the presidency was reserved for a Maronite Christian, the dominant Christian sect in Lebanon, while the prime ministership was allotted to a Sunni Moslem and the speakership of the parliament to a Shi'a Moslem.

Even as my Lebanese friends tried to convince me of the "genius" of the arrangement, the system appeared as nothing but a social time bomb. Instead of forging unity, the pact only institutionalized ancient cleavages, cutting out from key positions of power andbles a patronage whole groups, such as the Shi'as, who have since grown to be the nation's largest religious group.

In the decades since the 1932 census was taken, the demographics have changed, and it is now the Moslems who are the country's majority. The system set up in 1943 only perpetuated the dominance of the Maronite Christian minority over a growing and restless Moslem population, increasingly drawn into the volatile politics of its fellow Arab Moslem nations in the region.

The basic contradiction in aims between Christians looking west and Moslems looking east had already brought forth civil war in 1958. It was stilled only after the United States landed a force of Marines on Beirut's southern beaches in an action that was a precursor of today's presence of 1,200 U.S. Marines in Lebanon as part of an international peacekeeping force.

But it was in the 1970s that the system self-destructed. Under the strains of the regional Arab-Israeli conflict, the pressures of the PLO's growing strength in their country, and its own glaring internal contradictions, the casual violence that greeted me when I first went to Lebanon soon became the dominant tool for political and social discourse.

Turfs were carved out by militias around city neighborhoods, gun battles in the streets became ubiquitous, and assassinations a regular occurence. Violence, like the weather, became something one simply learned to live with.

Like a terminal cancer patient, Beirut -- symbol of the nation -- has simply withered before the eye. Once-graceful buildings were first pock-marked by bullet fire, then, as the firepower of discourse increased, holed by rockets and artillery shells. Other buildings simply disappeared into piles of rubble, blown up by car bombs or, more recently, Israeli dive bombers. Streets that once flourished with shoppers and traffic were transformed into corridors of fear where snipers and rats living amid uncollected garbage piles vied for control.

Returning year after year, I was confronted with the sad realization that whenever one thought things could never get worse in Lebanon, they did. By this year I could only describe Beirut in terms of what the world might look like after a nuclear holocaust: shattered ruins, burning fires, terrorized people, armed anarchy.

The open civil war that finally erupted in 1975 only confirmed what everyone already knew: Lebanon, the old Lebanon that tried to forge a state out of the violent diversity of its people, was dead.

The savagery that Christians inflicted on Moslems (and Palestinians) in places like the refugee camp at Tel Zataar and the counterterror that Moslems (and their Palestinian allies) inflicted on Christians in places like the sacked town of Damour, south of the capital, sealed forever any notion that the Lebanese could live civilized lives in harmony with each other.

The notion that order can somehow be imposed from outside has also proved fallacious. Syria tried to do so in 1976 with a 30,000-man army sent to end the civil war under an Arab League mandate. The violence only changed character. Warfare merely became sporadic, and assassination the preferred means of dealing with opponents.

The Druze leader Kamal Jumblatt, leader of the nation's left, was killed in such a way by unknown assassins, thought to be in the pay of the Syrians. Tony Franjieh, the old president's son and heir, also fell -- with his wife and two-year-old daughter and 30 retainers -- in a hail of gunfire ordered by his Christian rival, Bashir Gemayel, who himself was killed last month in a bomb blast eight days before he was to take office as president.

Israel has now entered the fray openly to try to impose order and stability in Lebanon where Syria failed. Its effort has hardly proved a success. It pinned its hopes of Bashir Gemayel, whose militia Israel trained and armed, but Gemayel was assassinated and replaced by his weaker, less pro-Israeli brother. The massacre of Palestinian refugees by Gemayel's out-of-control militia has also bbles arought home to Israel the grim truth that occupations in Lebanon are as dangerous for the occupier as the occupied.

The 4,000-man international peacekeeping force intended to keep Lebanese from each other's throats may provide a temporary pause in the violence. But no one really believes that when the international force is finally withdrawn -- be it in a month or six -- the Lebanese character will somehow be transformed.

Lebanese being Lebanese, their millennia- old hatreds will live on. The differences are as irreconcilable as the means of expressing them are violent. A nation requires a mutual identity, and that is precisely what Lebanon has never had. It has always been a collection of rival clans, tribes, religions, with only a common geography to force their uneasy coexistence. It is inconceivable, given all the new blood spilled in the past decade, that this will now change.

The new president of Lebanon has spoken grandly of reconstructing "the fiber of society by consolidating the spirit of unity toward which the Lebanese aspire, irrespective of political, religious or ethnic differences." But so far he has not even put together a government, much less prove that he can rule more than a few square miles around the presidential palace at Baabda that were his predecessors' limits of national authority.

With a weak army that in the past has split along confessional lines when put to the harsh task of trying to impose state authority, with his dead brother's ruthless Christian militia proclaiming itself an independent political and military force in the land for the foreseeable future, and with the demons of his nation's bloody history hovering everywhere, it is hard to conceive that the future of Lebanon will be that much different from its past.

Power balances undoubtedly have shifted for the moment in favor of the Christian Phalangists, but the hatreds and suspicions remain and will flourish anew. In short, the crisis in Lebanon is not over: It is merely in transition toward some new uncertainty and chaos.