Beirut — Unless there is a dramatic change in policy, the United States is virtually trapped by the Reagan administration's diplomatic initiatives to first sort out Lebanon, and then resolve the 35-year-old Arab-Israeli conflict.
In the process, US personnel will continue to be ''on the frontline,'' say Western envoys. The comments came after Sunday's bombing which killed more than 200 marines and which prompted a US reexamination of its role in Lebanon.
The envoys say the US cannot afford to pull out the 1,600-man marine contribution to the four-country multinational peacekeeping force without losing credibility for its peace initiatives, and appearing to surrender to the pressure of opponents.
In many ways, the Syrian government of President Hafez Assad holds many of the trump cards in a game involving US lives, since Damascus has the power and leverage to call the shots on all three major stages:
The first step is reconciliation talks between the Lebanese government, supported by the US, and opposition forces backed by Syria. US officials have conceded this may take many months of tortuous negotiations.
But even if there is a resolution, it is likely to be a considerable time before a new central government and its Army are strong enough to stand on their own without the psychological and physical support provided by the forces from the US, France, Italy, and Britain.
The second phase involves mediation to win the withdrawal of the estimated 70 ,000 Syrian, Palestinian, and Israeli forces occupying 80 percent of this tiny state.
The Assad government continues to balk at having anything to do with the US-orchestrated accord Lebanon signed May 17 with Israel on terms for the pullout of its troops. And there have been strong indications for many months that Damascus will use the issue of its presence in Lebanon to win bigger stakes , such as return of the Golan Heights captured by Israel in the 1967 war.
This step is also expected to be agonizingly long, and to involve alterations in the approach of US policymakers.
The final phase is talks on the Palestinian issue between the Arabs and Israel. Although the Reagan administration had pinned its hopes on the moderate Jordanian government of King Hussein, it increasingly appears that the militant, pro-Moscow Assad regime controls the pathway to peace on the Arab side.
Mr. Assad has raised his position in the wake of the mutiny in Lebanon's Bekaa Valley within the Palestine Liberation Organization. The rebels who weakened, if not totally overthrew, PLO chairman Yasser Arafat owe their new power almost completely to the support of the Syrian leadership.
Now it is Mr. Assad, not King Hussein, who can claim to be able to stand in negotiations on behalf of the PLO. But the price for Syrian agreement again is likely to be steep, and uncomfortable for the US.
National security adviser Robert McFarlane, the former Middle East envoy, appears to have recognized - just weeks before the Marine bombing - the crucial role Syria will have to play in all settlement efforts. Despite the anti-Syrian comments of Mr. Reagan, senior US officials had openly begun to acknowledge that Damascus had legitimate concerns and an important role in peace efforts.
The bombing of the Marine complex may serve to reinforce that feeling. For, as other members of the multinational force countries point out, if the US ever hopes to extricate its forces, or at least end the physical threats to its personnel emanating from the current trouble, American mediators will have to consider, and probably concede to, Syrian conditions.
Meanwhile, just 48 hours after the massive explosion in Beirut, US diplomats and American marines were put on maximum alert Tuesday because of new threats in the Lebanese capital. The latest scare reflected the increased vulnerability of any US presence in Lebanon, where more than 200 Marine and Navy personnel and 16 diplomatic staff have been killed in bombings over the past six months.
The scene at the Marine base at Beirut's airport was tense, as troops scrambled into bunkers or crouched behind antitank and machine guns repositioned after a warning that three trucks full of explosives were driving in nearby neighborhoods. Only rescuers still digging through the ruins of the bombed-out battalion headquarters were allowed to move.
On the other side of town, all ''non-essential'' personnel at the tightly guarded temporary sites used by US diplomats, including the British Embassy, were told to go home until further notice, following fears of a simultaneous assault on other American posts. The fears were raised because of a suspicious car.
The dual alerts emphasized current fears of repeated attacks against American officials, military or political, until the Lebanese crisis is finally settled - still many stages, and maybe many years, away.