An ominous split has developed during the past two months between Washington policymakers and United States personnel - military and diplomatic - in Beirut over strategy in Lebanon.
In effect, ''hawk'' and ''dove'' camps have evolved from the differing interpretations of facts on the ground here, according to US and multinational force (MNF) sources.
There is concern here that hawks in the Reagan administration - led by national-security adviser Robert McFarlane - will get the United States more deeply and dangerously trapped in Lebanon, ignoring the advice of US Marine and Army personnel and the US Embassy that the recent escalation of verbal threats and displays of military strength will only lead to a direct confrontation with Soviet-backed Syria rather than fend off future attacks.
The second danger is that the future of Middle East peace efforts will be endangered by saber-rattling potentates in Washington unable to keep their diplomatic cool during the most fragile stage of negotiations on Lebanon, upon which all else hinges, according to officials from all four MNF countries.
There is deep concern now in Beirut about conflicting versions of recent events, and how they are being used to exploit opposing strategies that could seriously influence the depth and length of the US presence in Lebanon, and the vulnerability of US personnel.
The rift has been particularly visible on four fronts:
1.The Damascus announcement that Syria's air defense system had fired on four US F-14 fighter-bombers last week, chasing them from Syrian-controlled eastern Lebanon back to their aircraft carrier in the eastern Mediterranen.
At the time, speculation centered on whether the American war planes were scared away by Soviet-made MIG fighters or the new SAM-5 ground-to-air missiles, so controversial when they were installed in January and still manned by Soviet technicians.
Beirut papers bannered it as the first direct confrontation between the US and its Mideast Nemesis - Moscow-supported Syria. And on Sunday, Mr. McFarlane said that US intervention in Grenada demonstrated US determination to react when threatened, referring specifically to the possibility of US planes in Lebanon being fired atagain.
But US sources in Beirut claim the incident was overblown by Washington. They claimed the F-14s were fired at only by antiaircraft missiles on the ground, and that it was not the first time. A Western military source said US planes were shot at ''quite regularly. It's never been considered a very big deal. They're not very good shots.''
The incident coincided with a dramatic increase in overflights by US warplanes, from two or three each day to dozens, reportedly ordered by Washington in the aftermath of the bombing that killed 239 marines.
Military sources acknowledged that Damascus, fearful after repeated US pledges of retaliation, would view the escalation as either clear-cut provocation, or preparation for some kind of attack. Several officials interpreted the Syrian announcement as a means of showing that it would not be intimidated by ''US acts of aggression.''
Key US officials in Beirut were dismayed when Washington did little to clarify the incident and defuse the heightened tension. ''Maybe that's exactly what they want,'' one noted.
2.Serious discussions in Washington about a preemptive strike to knock out a battery of Soviet-made multiple rocket launchers recently positioned near the Marine base and under control of pro-Iranian Lebanese factions.
One US television network recently reported that Pentagon sources said the strike could be carried out by ground units, listing either French commandos already in Beirut or by an American counterterrorist force, before the new weaponry can be used against the US marines.
Indeed, new BM-21s have been sighted in Borj el Barajneh and Hay al Salloum, strongholds of Shiite gunmen that border the Marine base.
But high-level Western military experts say the rocket launchers are, ironically, too close to be effective against the marines, unless they were shot straight up in the air, with hopes they would come down on the US force and not on themselves.
There is growing evidence to substantiate claims by US military and diplomatic personnel that the rocket launchers were positioned there by the Shiites for use against the Lebanese Army or rival Christian militiamen, in the event of attack or the breakdown of a ceasefire being honored for the reconciliation negotiations.
They have also warned that any assault by French or US troops would lead to a major bloodbath for both sides in the most densely populated section of the Lebanese capital, as well as a disaster politically for reconciliation efforts and the US role.
3. Washington's massive build-up of US military presence on and off the Lebanese coast, the largest since the 1973 Middle East war, involving 30 warships and three aircraft carriers.
The build-up led Syria last week to mobilize its entire force of 100,000 reservists, warning of ''US troop concentrations aimed at mounting an attack on Syria.''
US officials in Beirut have tried daily to put down reports of a major increase, pointing out that the armada will be there for only a few days.
As the marines carry through their long-scheduled rotation, ironically, their strength on land is actually now down to 1,400, the lowest in months, because of the evacuation last week of an entire company and two key checkpoints on the exposed front-line perimeter.
What irks key Marine officers is that Washington is talking big, while in fact ordering the reduction onshore to cut back on possible casualties - when the local force feels they should do the opposite.
The evacuation amounts to abandonment of front-line positions, which provided a buffer for the bulk of the corps stationed closer to Beirut airport.
''They're diluting our strength and our means for self-protection,'' grumbled one officer. ''Rather than avoid casualties, this just opens the way for a real live ground assault. It's another dictate from Washington. They're interfering again.''
4. The open boasts of threatened retaliation, which American military and diplomatic personnel in Beirut feel are highly counterproductive, putting them in an even more precarious position.
''Our main mission here now,'' lamented one official, ''is simply to avoid getting blown away.''
Few American and MNF officials feel there will ever be sufficient evidence to know which party, ultimately, was responsible for the blast. This, despite new evidence, including the names of the suicide drivers of the two bomb-laden trucks.
(A top defense official told the House Foreign Affairs Committee Monday that the US may be unable to retaliate because the perpetrators may not be identified.)
The threats have only served, so far, to put all parties in the region on edge and to cloud the atmosphere needed to resolve the Lebanese crisis.
US officials trace the rift back to Sept. 19, when US warships opened fire on Souk al Gharb. The move was designed not to protect the marines, but to avoid the fall of the Shouf mountain town to anti-government militias.
Both the US Embassy and the Marine command structure advised against the move , reportedly ordered by Mr. McFarlane when he was new as the US envoy to the Middle East.
The Beirut staff argued that it would cost the US its neutrality, and its credibility as a peacemaker and broker among all sides. Many embittered officers feel Mr. McFarlane's decision was at least in part responsible for the bombing of the Marine headquarters five weeks later, because the moment the US contingent became involved on one side, it became a target for all the others.
Mr. McFarlane's subsequent appointment as President Reagan's national-security adviser has increased fears of a more militant strategy in Lebanon. In the eyes of local personnel, that could too easily backfire on a much smaller but equally dangerous scale as Vietnam.