The first contingents have arrived in Beirut of the 2,000-to 3,000-member multinational force, its objectives unquestionably well-meaning but its operational plans vague and its length of stay uncertain.

President Reagan has said that the men will not "act as a police force," by which he presumably means they will not patrol the perilous streets of the Lebanese capital, and that they will be withdrawn after a "limited period of time."

He is wise to say no more, for the mission of the force must surely be clarified on the scene. But if his intention is, as he says, "to make it possible for the lawful authorities of Lebanon to discharge (their) duties," not only while the force is there but also after it has gone, then we had best be prepared for dangerous work ahead, and a stay of long duration.

Let us look for the reasons past the horrors of the last 10 days in the Palestinian refugee camps and back to the recent civil war. During the period from April 1975 to the end of 1976, by best estimates 60,000 Lebanese were killed, out of a total population of 3 million. That is a ratio of dead of 2 percent, which, transferred to an American context, would be 5 million people.

A large proportion were "identity-card murders," in which unarmed victims were selected strictly for their religious affiliation. As often as not, the killings were accompanied by incredible savagery -- mutilation, rape, torture. When asked about their conduct, the killers normally had no better explanation than, "They started it."

Without notable success, scholars have tried to understand Lebanon's descent into barbarism. They have been a little better at explaining the breakup of the Lebanese state into seemingly irreconcilable factions, each too fearful or religiously self-righteous or ideologically rigid to live comfortably with its neighbors.

Throughout most of its history, Lebanon was saved from this sort of deadly factionalism by the tight rule of an occupying power. The most recent, the French, left Lebanon independent after World War II and, for a time, the factions maintained an uneasy peace under a constitution written in Paris.

But these were the years in which the Christians grew rich while the Moslems raised babies, and tensions steadily intensified. In 1958, the Lebanese fought a small civil war, brought to an end by the first American occupation. In 1975, the country erupted into the chaos in which it lives to this day.

The Palestinians are much blamed for this chaos, and, in importing heavy weapons for their war against Israel, they did indeed upset the fragile military equilibrium among the factions. Scores once settled by knives and pistols came, with the escalation of arms on all sides, to be settled by mortars and even tanks. This alone helped to trigger the civil war.

The PLO did even more in lining up with the Moslem left, bringing it into confrontation with the Christian right, and among the early battles were those in which Christians opposed the PLO. But after suffering heavy losses, the PLO decided it was fighting the wrong enemy, and by 1977 it had largely withdrawn from the war. It became something of a stabilizing force among the factions. The chief stabilizers, however, were the Syrians. Though it became fashionable in Washington to talk of them as trouble-making "foreign" forces, in fact the Syrians alone since 1976 kept the Lebanese from slaughtering masses of one another. They were rewarded by the Arab world with money and prestige, but it remained a dirty job they did, which no one else was able to do.

The Begin government was arrogant to think it could drive out the PLO and the Syrians, then assign to the Christians the task of keeping the peace in Lebanon. It based its entire invasion upon the illusion that the Christians could uonite the country. The massacres in the Sabra and Shatila camps revealed how wrongheaded that illusion was.

In the past year, I had conversations with the men who were then leaders of the Moslem left and the Christian right, Walid Jumblatt and Bashir Gemayel respectively, and they said the same thingh absolutely no progress had been made since the bloody days of 1976 to reconcile the contending factions.

Logically, it would make sense to give up on Lebanon as a nation, and partition it into segments. But nobody seems to want that, not even the Christians, though they would have their own state. Partition would be an invitation to the Syrians to grab the east, the Israelis the south, then to face each other eyeball to eyeball. That would add to no one's security.

The advantage the American-French-Italian force will enjoy over its predecessors is manifest disinterestedness. It could provide the calm which, mixed with judicious meditation, hostile parties need to sit down together. Though the odds are long, a Lebanese nation might conceivably be pasted back together.

But a long time will be required, and I suspect our forces will take a few casualties along the way. I wonder whether the American people and the White House, whoever may be in residence, will have the patience to wait.