BEIRUT ON THE BRINK; Balance of power shifts in Lebanon

The new explosion of violence around Beirut provides a freeze-frame look at the changes in Lebanon's power balance - favoring the Muslims, the Druzes, and Syria, and spelling potential trouble for American policymakers.

Syria's is the key voice.

By the time Lebanon's cornered Christian President, Amin Gemayel, announced a shoot-on-sight curfew in Beirut Monday afternoon, the changes had been a long time in the making.

The new fighting - and Sunday's resignation of Muslim Prime Minister Shafik Wazzan - simply made the new power balance official.

The Reagan administration faces a difficult choice.

The American President, facing election year pressure, can either bow to Syrian pressure for speedy withdrawal of United States Marines or leave in place a force incapable of forcing compliance with the stated American objective of a stable, sovereign state of Lebanon free of either Syrian or Israeli control.

Increasingly, the most likely compromise for all sides seems one in which Syria and Lebanon's Muslim majority would trade some gradual, face-saving arrangement of US and Israeli troop withdrawals for on-the-ground recognition of the new balance of power in Lebanon.

In other words, this would mean a new Beirut government with a greater voice for the Muslims, and a security setup giving the Syrians an influential say in their neighbor state's relationship with the outside world, notably with Israel.

Whether the Syrians would withdraw their own 40,000 troops from Lebanon under such an arrangement is difficult to predict. Most Mideast analysts assume at a minimum that Israel would first have to pull back its 1982 invasion force from southern Lebanon.

And many analysts feel Syria would, in any case, want to maintain a military presence in eastern Lebanon's Bekaa Valley.

President Gemayel is caught in the middle.

He has been made more and more keenly aware since late last year that Syria could make or break any attempt to reassert control over his country. At last November's Lebanon reconciliation conference in Geneva, Mr. Gemayel began an ultimately unsuccessful attempt to patch up his relations with Damascus. He hoped he might then get the US and Israel quietly to shelve last May's accord envisaging simultaneous withdrawal of Israeli and Syrian forces.

US negotiators had pointedly ignored Syria during the months-long mediation process leading to the Lebanese-Israeli agreement, seeking Syria's assent only after the pact was sealed.

''The thinking at the time was that to bring Syria in on these negotiations at an early stage would simply drag the process out . . . ,'' explains one US official. ''This was undoubtedly true. But in hindsight, my personal feeling is that the exclusion of Syria was a major mistake.''

Back-to-back visits to Washington in December by Israeli Prime Minister Yitzhak Shamir and by Gemayel promptly dashed his hope of outside help in finessing the issue of the May 1983 accord - which, the Syrians note, envisages eventual ''normalization'' of Lebanon's ties with an Israeli ''enemy'' that invaded Lebanon and still occupies a slice of Syria (the Golan Heights) captured in the 1967 Mideast war.

First Mr. Shamir, then the Americans, publicly reaffirmed the May 1983 agreement in the days before Gemayel's arrival in Washington.

Syria, aware of political pressure within the United States and Israel for a withdrawal from Lebanon, reaffirmed its demand that the May accord be scrapped.

But the issue of the May agreement is a metaphor for the larger question of Lebanon's political future.

This has become abundantly clear in the run-up to the current Beirut crisis - with opposition Druze leader Walid Jumblatt, for instance, posting intermittently amended conditions for internal political entente even as a weakened Gemayel regime hinted at meeting at least some of them.

Nabih Berri, the main leader of Lebanon's Shiite Muslims - the country's most numerous and disadvantaged community - has similarly hardened his stand in recent days.

Neither Mr. Jumblatt nor Mr. Berri seems yet to envisage the utter dismemberment of the complex 1943 power-sharing accord whereby Lebanon's President would come from the country's then-majority Maronite Christian community.

And both Muslim leaders ''are no doubt aware that Syria's predominant military sway ensures Damascus a key say in what they demand, or achieve. Yet both men - and Syria - want at least a far greater representation in the country's government and armed forces for non-Christians.

Still, even if Mr. Gemayel decides on a renewed approach to the Syrians, he must heed an additional political force: his own Maronite Christians, and especially the predominant Phalangist party which his father, Pierre Gemayel, heads.

The Phalangists appear increasingly outgunned on the ground.

They cannot be sure, moreover, of any effective intervention from Israel in the struggle unfolding in Beirut. Israeli officials Monday said as much, stressing that their own primary concern was the Israeli-occupied south of Lebanon. Mr. Shamir went so far, in a local newspaper interview, to suggest that if acceptable security arrangements for Israel in that area could not be worked out with the help of the Gemayel government, Israel would move to tackle the task on its own.

But from the Phalangists' point of view, the aim of the civil strife that began in 1975 was to reaffirm Maronite Lebanese control against ''foreigners'' - first Yasser Arafat's Palestine Liberation Organization, and then the Syrians.

It is not at all clear that the Phalangists - particularly the Phalangist militia force made up largely of fiercely determined fighters who got their start as teen-agers in the 1975-76 civil war - will be ready to abandon that battle from mere mathematical calculation of a changing balance of power.

Gemayel's next move may necessarily be determined as much by his evaluation of the Maronite share of power as by that within Lebanon as a whole.