Why UN peace force offers no easy way out for US Marines

It looks so simple. You just pull the Marines out of Lebanon and plug in a United Nations force instead. Presto. Problem solved.

Americans in their offices and congressmen in their resolutions keep coming up with this apparently easy way to ''bring the boys home.'' It gains adherents with every Marine casualty - and the toll has just edged up again.

Would that it were so simple. The truth of the matter is that Syria's President, Hafez Assad, has a double veto over such a face-saving American exit.

A visit to New York's diplomatic circuit, which revolves around the soft-carpeted corridors and lounges of the United Nations, removes any doubt that there is no easy ''out'' for the Reagan administration. Indeed, many senior diplomats of varied nationality and political persuasion are baffled at how Washington ever got itself into such a box in the first place.

The conventional diplomatic wisdom goes like this:

President Reagan has said he will not withdraw American troops from the US-French-Italian-British multinational force (MNF) unless either total chaos or some form of political stability intervenes. To ''run away'' from Beirut, it is widely agreed, would do serious damage to the Western position in the Mideast as a whole.

But is it possible to stitch together even the most modest agreement among Lebanon's factions without President Assad's say-so?

The Syrians have more than 40,000 troops in eastern Lebanon. Their current Lebanese ally is Walid Jumblatt, leader of the Lebanese Druze.

With these two large-size pieces on the Lebanese chessboard, President Assad can check any move toward the sort of internal Lebanese compromise that might allow the Americans to withdraw with any semblance of honor. That's veto No. 1.

In addition, if a UN peace force is to be slipped into Beirut in place of the current MNF, the UN Security Council will have to agree. The Soviet Union has a veto in the council. There is little doubt at the UN that, if the Syrians ask, the Soviets will deliver a resounding ''nyet.'' Hence veto No. 2.

What's more, few governments are ready to offer peace forces for service in the current maelstrom. They don't want to repeat the US experience. The Reagan administration, say diplomats experienced in peacekeeping, ''has completely disregarded all the principles of peacekeeping.''

The contrast with former Secretary of State Henry Kissinger is frequently made. Kissinger's shuttle diplomacy, it is said, carefully included all the main actors, including Syrians and Russians. He followed a coherent strategy. He avoided a tilt toward one side or another - such as the present US ''strategic cooperation'' with Israel. He worked out a deal before asking for a peace force to police it. The resulting force was strictly neutral, integrated, low key, and equipped essentially with side arms only.

So what can be done? First of all, even Washington's closest allies agree, you have to bring the Soviets and the Syrians back into the process. On that, a tentative start has been made.

Secretary of State George Shultz and Soviet Foreign Minister Andrei Gromyko are said to have discussed the Mideast last month in Stockholm.

In a separate meeting, French Foreign Minister Claude Cheysson reportedly probed Mr. Gromyko specifically about inserting a UN force, perhaps shifting the current UN force in southern Lebanon northward. Gromyko apparently avoided giving an outright ''no'' by responding with a string of questions.

On April 19, the Security Council is due to renew the mandate for that UN force in southern Lebanon. Could a first step be taken, though still in the south, by redeploying it to fill the gap created by any further Israeli pullback? That, too, however, requires some increase in Lebanese political stability.

US envoy Donald Rumsfeld has been in Damascus this week for another round of talks with the Syrians. This at least puts President Assad visibly back in the picture, gives him added ''face.''

But so far there has been little sign of Syrian give on the central symbol of contention: last spring's American-sponsored Lebanese-Israeli pact, which Syria says allows the Israelis too dominant a role even after a troop withdrawal.

The Lebanese might like to forget it. But the Americans and Syrians have nailed it to the mast. Hence, with domestic political pressures mounting on Reagan (and his MNF partners) to withdraw regardless, Assad's veto remains in effect - and the marines under fire.