The United States and Syria are emerging as key to the immediate future of an unstable Middle East - and of a particularly unstable Lebanon - as other traditional protagonists gaze expectantly on.
If weaker militarily than the Americans, Syria seems at least for now in the more comfortable position politically.
Adding to the general air of expectancy by Monday was the reported hospitalization of Syrian President Hafez Assad, delaying what would have been the first visit to Damascus by Lebanese President Amin Gemayel since he took office in September 1982. The illness also seemed to rule out any early meeting with Mr. Assad for the new US envoy to the Mideast, Donald Rumsfeld.
After a week in which the US beefed up its Mediterranean fleet to unprecedented proportions, and the Syrians publicly announced a full military call-up, tensions had, by Monday evening, seemed to abate a fraction.
The Americans had begun to speak less explicitly of ''retaliation'' for last month's bomb attack on the marines in Beirut - a counterstrike which, public remarks suggest, would most likely target the suspected bases of extremist Shiite Muslims in the Syrian-controlled eastern sector of Lebanon. And Syria's public announcement of its call-up was seen as suggesting Syria was more interested in forestalling a US strike than in countering one.
Still, all the elements of confrontation remain. If it occurs, a main victim could be embryonic efforts at political detente within Lebanon.
Monday, US reconnaissance planes overflew Beirut - this, after an incident last week in which similar jets drew surprise antiaircraft fire from Syrian positions elsewhere in Lebanon. And for the first time in some two months, Lebanese Druze militiamen opened shellfire from outside Beirut on the Christian-controlled part of the capital.
The USSR, Syria's formal superpower ally, has its own leader's ill health to worry about. When Syrian Foreign Minister Abdel Halim Khaddam visited Moscow last week, his Soviet counterpart evidently voiced displeasure over Syrian backing for Palestinian dissidents besieging the forces of PLO chief Yasser Arafat in the north Lebanese city of Tripoli.
Soviet news reports of the Khaddam visit also omitted any explicit Soviet pledge of backing for the Syrians in the case of confrontation with the Americans. But at least at present, the Soviets would seem to need Syria at least as much as the Syrians need Moscow. Syria is the Kremlin's major Arab ally.
The Palestine Liberation Organization continues, meanwhile, a loss of regional influence that began with Israel's summer 1982 invasion of Lebanon. As part of the initial cease-fire, Mr. Arafat and the majority of Palestinian guerrillas abandoned their base in Beirut. Since then, a hard-line faction has challenged Arafat's PLO leadership. Now Arafat's back is literally against the sea, and his departure from Tripoli seems only a matter of time.
Israel has been remarkably quiet in recent days. Only Monday did the country's Cabinet get around to discussing the implications of Syria's military alert. While citing a general Syrian ''threat,'' senior Israeli officials have been publicly playing down the prospects of war with Damascus. Increasingly, Israel's long-term concern seems to be to find a way to arrange an honorable withdrawal of at least most of its troop strength in Lebanon.
''The Israelis at present seem to be waiting to see what the Syrians - but, most of all, what the Americans - will do,'' said one Western diplomat. Israel also is eyeing Arafat's fortunes, which could affect the Israelis' prospects of finding a way out of southern Lebanon - even, conceivably, via the kind of tacit understanding with Syria that facilitated Syrian forces' initial entry into Lebanon after that country's 1975-76 civil war.
The central factor in the entire equation is Syria. This is an odd turn of events given the immediate aftermath of Israel's military drive into Lebanon 17 months ago. With Israel's demonstration of superior military force, Syrian President Assad withdrew his Army from Beirut after a six-year presence. Syria's Air Force and its installations in eastern Lebanon had also taken a beating from the Israelis. The US expectation was that Mr. Assad could be brought around to withdrawing all troops from Lebanon as part of a deal by which Israel would do the same. The theory was that a pro-Western government, under the dominant Gemayel family, would then reunite and rebuild the country. And US marines would triumphantly sail home.
But the Syrians said no. They rejected a troop-withdrawal accord between the Lebanese government and Israel. Indeed, they rejected President Gemayel's legitimacy as Lebanese ruler, and they reinforced links with various anti-Gemayel forces in Lebanon.
One major development in the first round of the Lebanon reconciliation talks earlier this month seemed a fresh realization among Lebanon's rulers that Syria is a key to any workable national entente.
The US view of Syria remains unclear, although US national-security adviser Robert C. McFarlane did caution Syria against further firing on US jets. The Americans have been muting their public blasts at Damascus somewhat in recent weeks. Some Mideast analysts see this as a sign that the US, too, may have come to see Syria as the key to extricating its marines from Lebanon.