Beirut — President Amin Gemayel has finally decided on the so-called ''Arab option.'' By flying to Syria Wednesday, apparently ready to tear up Lebanon's US-orchestrated pact with Israel, the Lebanese leader has moved sharply away from American efforts to secure peace in the region.
And, as diplomats were quick to point out, the trip also means Syrian President Hafez Assad has won another round in the regional struggle to establish influence in Lebanon, defying two years of effort by the United States to squeeze Damascus out of the picture.
There is strong evidence that turning to Syria was Mr. Gemayel's least-favored option. Diplomats and local officials say Gemayel was forced to turn to Syria - the Soviet Union's closest ally in the Mideast - after the US and Israel turned down requests over the past two weeks to help his beleaguered regime hold off further assaults from opposition forces. Two presidential advisers dispatched to Jerusalem and Washington reportedly were bluntly told there was no enthusiasm for fresh efforts to get more deeply embroiled in the Lebanese quagmire.
Gemayel's visit, his first to Syria since he took office in September 1982, was the first step of a Saudi-mediated plan to end nine years of civil war by reconciling the Christian-led government with the Syrian-backed opposition.
Lebanese officials said Assad and Gemayel would discuss:
* Stabilization of a cease-fire between government forces and Muslim and Druze militias, including the possibility of deploying United Nations troops to replace the mostly departed multinational force.
* Normalization of relations. This is significant because Syria has never formalized diplomatic ties with the territory that was historically part of ''Greater Syria.''
* The Israeli-Lebanese troop withdrawal accord signed May 17, 1983, and the Israeli occupation of southern Lebanon.
* The presence in eastern and northern Lebanon of some 40,000 Syrian troops, who arrived in 1976 as a peacekeeping force under an Arab League mandate.
* National reconciliation efforts to change the power-sharing formula between minority Christians and majority Muslims, which would pave the way for a new government.
Although it is now accepted that Mr. Gemayel will follow through with a formal cancellation of the so-called May 17 accord, Lebanese sources claim he does not want to act without guarantees from Syria first that Mr. Assad will agree to pull out his troops.
The news flash on local radio stations of the President's midmorning departure evoked almost visible sighs of relief around the capital. After months of procrastination and stalling on various US and Saudi proposals, Gemayel was seen to be finally doing something concrete to end the escalating crisis. The delays most recently cost him half the capital, and mass defections from the Lebanese Army.
But Gemayel's journey to Damascus is by no means the end of his troubles, which are certain to take, at a minimum, many months to unravel.
Cancellation of the May 17 pact will mean that there is no provision for withdrawal by Israel of its estimated 25,000 troops, which now occupy one-third of Lebanon. Israel has already announced that it will take matters into its own hands, including possibly prolonging its stay, if the accord is abrogated. This has triggered renewed fears that southern Lebanon will become, de facto, the ''north bank'' of Israel.
It is also doubtful that President Assad will agree to withdraw his forces before Israel, even if he is prepared to pledge an eventual withdrawal in principle. He has consistently opposed withdrawing his forces since Israel's 1982 invasion.
That would not only lead to a stalemate, and de facto partitioning of Lebanon , but could also affect formation of a new cabinet to replace the government that resigned more than three weeks ago.
Former President Camille Chamoun, leader of the Christian National Liberal Party, again warned Gemayel of a boycott by Christian dissidents unless he wins a disengagement agreement from Damascus.
The strength of the Christian fear was evident Tuesday at a rally of some 8, 000 Christians in Jezzin, who demanded closer ties with Israel. Large banners declared, ''Yes to Israel, yes to peace. We reject the Syrian presence in Lebanon.''
Although the Phalange Party, headed by President Gemayel's father Pierre, and the influential ''Maronite monastic order'' have both said over the past week that they would support any decision taken by the President, Christian dissidents are still strong enough to play a spoiler role.
Fady Frem, commander of the increasingly independent Phalangist ''Lebanese Forces'' militia, has already defied the party hierarchy. Over the weekend he said, ''We are opposed to cancellation of the accord because this would mean submission to Syrian control.'' He has also threatened to take military action if Lebanon abandons its Israeli ties.
Another point of concern for Gemayel is whether the Damascus trip and the cancellation of the May 17 accord will be sufficient to end calls by Druze and Shiite Muslim factions for his resignation. He will need maximum Syrian support to wield leverage over the opposition to abandon that condition so they will attend a resumption of reconciliation talks in Geneva.
Opposition forces were euphoric Wednesday when the Druze ''Voice of the Mountain'' radio station editorialized about the President's trip:
''It will be a major victory for Lebanese nationalists and Syria, which defied the American-Israeli-Phalangist policy and toppled it, providing with its allies in the Lebanese opposition that Lebanon cannot but be Arab.'' The May 1983 Israeli-Lebanese accord
Essence: Calls for Israel to withdraw troops from southern Lebanon and allows Israel to continue policing the area. The accord is a step less than a full treaty. Accord goes into effect only after Israelis withdraw.
When and where: Signed May 17, 1983, at ceremonies in Khalde, Lebanon, and Kiryat Shemona, Israel.
Terms of agreement:
* Pledge to ''live in peace with each other'' and to ''declare the termination of the state of war between them.''
* Establishment of security zone in southern Lebanon, patrolled by at most eight Lebanese-Israeli teams and two Lebanese Army brigades.
* Establishment of liaison offices in each country, if desired. While agreement does not establish diplomatic relations, such offices could be seen as embryos for embassies.
* Start-up of negotiations within six months after Israeli troop pullout. Normalizing movement of goods and people across the frontier.
Conditions for Israeli troop withdrawal:
* Stated under an accompanying understanding, if Syria and the PLO do not agree to pull troops out of the east and north of Lebanon, Israel will not be obligated to withdraw. The PLO is largely out of Lebanon, but Syria has rejected the agreement.
* Return of all Israeli prisoners of war and an accounting of Israelis missing in action.
* Secretary of State Shultz was a principal architect of the accord.
* US envoy Morris Draper participated in negotiations and signed accord as a witness.
* US to participate in Joint Liaison Committee to oversee carrying out the agreement.