With Lebanon again on the brink of full-scale civil war, President Reagan soon may have to choose between the politically risky step of increasing the number of U.S. Marines there or abandoning American attempts to help restore peace to that strife-torn country.

That is the unenviable decision that appears to be confronting Reagan a year after he decided to use the Marines in Lebanon as the cutting edge of a U.S. strategy to shore up the authority of President Amin Gemayel's government and prevent the country from being dismembered into enclaves controlled by Israeli and Syrian occupying armies and feuding Lebanese factions.

Now, however, this strategy is in danger of being engulfed by the renewed fighting that has swept through Beirut and the nearby Chouf Mountains this week. The Marines and others comprising the Multinational Force have neither the numbers nor the mandate necessary to control the situation, and the administration is coming under heavy pressure from the Lebanese government to try to restore the balance by expanding the size, scope and responsibilities of the international force, including its American component.

The Reagan administration, which saw two Marines killed in the fighting in Beirut on Monday, has been extremely reluctant to do this. Sending more Marines there could unleash a storm of public and congressional protests that Lebanon is becoming a new Vietnam and that the United States should avoid further involvement.

Administration officials are known to feel that, in the end, they probably could ride out the storm and get away with at least a modest increase in American involvement in Lebanon. But senior officials said that it could be done only at the cost of a bruising and divisive national debate affecting Reagan's long-range strategy in the Middle East and other areas of potential military confrontation like Central America.

So far, the administration has chosen to seek "no change" in American involvement, asserting instead that it believes the current fighting can be resolved through diplomacy such as the mediation among Lebanese factions now being attempted by Reagan's special Mideast envoy, Robert C. McFarlane.

That was the thrust of the letter Reagan sent Congress yesterday in hopes of staving off a debate that could lead to forced removal of the Marines under the War Powers Act. The White House, while saying the Marines would remain, made no mention of increasing their numbers and sought to minimize the situation by stressing that a cease-fire went into effect in Beirut last night.

Privately, though, many administration officials conceded that this is essentially a temporizing position that contains a much wishful thinking. These officials said the chances for a durable cease-fire are very fragile. They expressed fear that the situation could deteriorate to the point where the American, French, Italian and British units of the multinational force will be the only force capable of restoring order in Beirut.

What's more, they added, the international force won't be able to do the job unless it is made much larger and given a mandate to move from its present, essentially defensive positions into open confrontation with the various Lebanese militias shooting up the city. Otherwise, some of these officials predicted, the Gemayel government will be in peril of imminent collapse that would leave Lebanon without any semblance of central authority.

That is exactly the opposite of what the administration hoped to achieve last September when it surveyed the situation after the forced departure from Lebanon of the Palestine Liberation Organization and detected what it regarded as a "window of opportunity" through which to restructure the country after years of bitter civil war between its Christian and Moslem populations.

With Gemayel's Christian-dominated, fledgling government eager to accept American tutelage, Washington saw a chance to transform Lebanon from a major source of Mideast tensions into an American client state. The Lebanese armed forces would be trained by American advisers so that they could assert the government's authority throughout the country. In the meantime, the Marines, who had been sent to Beirut during the PLO evacuation, would remain as a confidence-building symbol of U.S. backing for Gemayel.

At the time, U.S. officials believed that their main problem would be to force the withdrawal of the Israeli forces that had invaded Lebanon in a massive drive against the PLO. In the U.S. view, an Israeli pullout would set the stage for a similar withdrawal by Syria and the remaining PLO forces under Syrian protection and permit Gemayel to seek reconciliation with the many factions holding his government at arms length.

The high-water mark of U.S. expectations came last May when Secretary of State George P. Shultz went to the Middle East and worked out an Israeli-Lebanese peace agreement. But the accord was derailed when Syria, on whose cooperation the administration had counted, rejected the pact as a "sellout" of Arab interests and refused to withdraw its troops from Lebanon.

The Israeli government, under heavy pressure to stop the casualties being suffered by its forces in the Chouf, then decided to retrench its troops in southern Lebanon. That would leave a power vacuum in the Chouf. So, in a grimly ironic reversal of his past efforts, Reagan this week was obliged to ask Israel to temporarily postpone its redeployment while McFarlane tries to work out an agreement between the Christian and Druze militias there for the Lebanese army to move into the area without opposition.

In the meantime, Syria has sought to undermine the Gemayel government by encouraging its many internal enemies to defy it with violence. Syria is believed by American officials to have played a big part in stirring trouble between the Druze and Christians in the Chouf. White House officials asserted Monday, while McFarlane was busy trying to calm the dispute, that the Syrians incited Shiite Moslems to the attacks that buffeted Beirut this week.

The convergence of these factors--Israel's decision to essentially disengage from the situation it created through its invasion and Syria's efforts to exploit the resulting vacuum--has dashed U.S. hopes for a relatively tranquil atmosphere in which Gemayel could impose his authority.

U.S. officials acknowledge that his military and security forces will be unable for at least another year to deal effectively with continued civil war. That leaves the international force, by default, as Gemayel's only hope for effective military backing.

But it is by no means clear that the other participating countries would agree to increase their involvement. Even if they were to cooperate, U.S. military leaders see the choices as equally unenviable.

To pull out would mean the downfall of Gemayel and the end of American credibility in the region. To keep the Marines there in their present size and role would leave them a target for continued attacks and casualties. And to increase the force with orders to keep the peace would put it in a combat role that seems certain to cause a major new debate in this country.