The fierce Palestinian civil war has highlighted the importance - and limits - of Soviet influence in the Mideast. Amid the lull in the fighting in the battered north Lebanese coastal town of Tripoli Tuesday, Farouk Khaddoumi, a top deputy of besieged Palestine Liberation Organization chief Yasser Arafat, flew to Moscow in what seemed his latest bid for Kremlin intercession.
Moscow's main asset, after a series of Mideast setbacks in the past decade, is its formal 1980 friendship pact with Syria.
The Syrians are the decisive backer of the dissidents within Mr. Arafat's Fatah guerrilla group, which has been closing in on his forces in Tripoli.
Mr. Arafat charged Monday he was also hemmed in from the sea by Israeli boats. Israel's main immediate interest is to ensure he does not leave without freeing six Israeli soldiers held since the 1982 war in Lebanon.
Thanks in large part to infusions of arms, command-and-control equipment, and personnel from Moscow since the 1982 war in Lebanon, Syria is stronger than it has been in many years.
The Syrians - a fact no doubt pleasing to the Kremlin - have played a key role in blocking President Reagan's bid to consolidate the power of a pro-Western regime in Lebanon and, in so doing, extricate the US Marine contingent. Damascus has also helped keep an at least equally vulnerable, and far larger, Israeli force bogged down in south Lebanon.
But the PLO showdown has revealed the complexities of a Soviet-Syrian relationship in which the Kremlin cannot call all the shots.
Syrian President Hafez Assad's inescapable reliance on Soviet military backing in any clash with Israel does give Moscow a large say in how far Damascus goes in its saber rattling on that front.
But Mr. Assad, a past master at walking a tightrope between the superpowers, seems keenly aware that, at least for now, Moscow needs him at least as much as he needs Moscow.
Since 1972, the Soviets have been booted out of Egypt. They have been all but excluded by the US from Arab-Israeli diplomacy. They have watched Mr. Arafat, despite longtime support from Moscow, court West European and US diplomatic favor, and briefly flirt with Jordan's King Hussein on accepting President Reagan's 1982 peace blueprint for the Mideast.
One Western Mideast expert remarks: ''Moscow really has little choice but to fall squarely behind the Syrians in any face-off with Israel.''
Mr. Assad - except for his questionable health - is in a stronger position than when he penned the treaty with Moscow three years ago. He had long resisted the idea of such a pact, partly because he held out hope that the US might mediate acceptable peace terms with Israel.
But Egypt then signed what amounted to a separate peace with the Israelis. Other Arab states - longtime rival Iraq, more moderate Gulf oil states like Saudi Arabia, and Jordan - began piecing together a post-Camp David alliance which threatened to leave Syria out in the cold. And Mr. Assad, who comes from the minority Alawaite Muslim sect in an overwhelmingly Sunnite Muslim nation, also faced internal unrest.
Syria's estimated 40,000 troops in Lebanon, as well as the Syrian's Air Force , took a bad licking during Israel's invasion in the summer of 1982.
But since then, Syria's military presence next door has given Mr. Assad a crucial role in Mideast diplomacy. Syria's backing of a rebellion inside Mr. Arafat's Fatah has succeeded in turning almost all eyes - US, Soviet, Jordanian, Saudi and, of course, Palestinian - in one way or another toward Damascus.
In not quite the way Moscow may have liked, the fighting has also given the Kremlin a Mideast diplomatic role it has long sought. Besides the visit of effective PLO ''Foreign Minister'' Khaddoumi to Moscow Tuesday, two recent statements by Fatah, in a clear bid to undermine or co-opt hard-line Palestinian rivals, called for ever closer ties with the Soviets. And last Friday, two senior French Foreign Ministry officials flew to Moscow - ''at the invitation and initiative'' of the Soviets, Paris said - to consult on the PLO dispute.
Moscow might benefit if the fighting produces a more hard-line and more reliably pro-Soviet Arafat. But the Soviets would gain far less from the emergence of a new PLO under a strengthened Syria, managing physically or politically to eliminate Arafat.
In recent days, Moscow seems to have been hiking efforts to rein in the rebels.
Nearly two weeks ago, the Kuwaiti News Agency reported the Soviets had sent envoys to Syria and to Libya, the other main Arab backer of the PLO rebels, to ask them ''not to interfere in what is happening between the supporters of Yasser Arafat and the dissidents.''
Soviet pressure may still help rescue Mr. Arafat. But the evidence of the last 10 days suggests Syria is not exactly scurrying to meet Kremlin wishes.