Deployment of Syrian antiaircraft missiles in eastern Lebanon, provoked by Israel's deep-penetration kills of two Syrian helicopters, has given Moscow a sharp wedge to pry itself back into the Mideast as a major player.

That drops the first case of crisis diplomacy into the laps of Secretary of State Alexander Haig and Vice President Bush, President Reagan's crisis manager, with precious few assets.

Indeed, given his unnecessary concessions to Israeli Prime Menachem Begin in the past three months, Ronald Reagan may find himself trapped between Begin's election-eve military operations in Lebanon and the intolerable countermoves of Soviet-backed Syria.

Although some U.S. officials dispute it, experienced Arab diplomats doubt that Syria's beleaguered President Assad would have dared to move those SAM-6 antiaircraft missiles into Lebanon without the consent of the Kremlin. To Begin, the SAMs naturally act as magnets for his U.S.-supplies air force.

Begin's popularity has soared on the wings of his daily air raids against undefended southern Lebanon. His program to implant new Jewish settlements on the Arab-occupied West Bank of the Jordan has now reached runaway status, openly encouraged by Reagan's new policy edict that the settlements are "not illegal." Besides strengthening Begin, another result is to make the United States look weak and flabby to its Arab friends.

For the Soviets, this lethal package is an unexpected bonanza that could shred the fragile, crazy-quilt cover of peace that Jimmy Carter's Camp David agreements spread over the Middle East in 1978. With quiet help from some influential European allies of the United States, the Soviets have begun to set their price for an affirmative response to the U.S. appeal that Syria be stroked and calmed. That price: a Soviet share in any overall settlement in that tortured region.

Haig has no illusions about the dangers of letting the Soviets play any such role. That is even truer now than when Henry Kissinger dealt Moscow out of the Mideast. Today there is no shah of Iran. Formidable Soviet military power is now on the peripheries: in Afghanistan, South Yemen, Ethiopia and Libya. Although Haig has not been able to withdraw or even diminish the political gift Reagan handed Begin with his reversal of a 13-year-old policy against West Bank settlements, there is no light between him and the president on the urgency of keeping Moscow's itchy fingers out of the Middle East.

That underlines a paradox in the administration's murky Mideast policy. By refusing to invoke U.S. law against Israel's offensive use of American-supplied aircraft in Lebanon and by giving a free hand to Begin's new settlements, Reagan unknowlingly may have unleashed the very forces that now offer the Russians their long-sought chance for a Mideast comeback.

Beyond that danger lies another: the unwinding of delicate threads that bind up the Camp David accords. If the contest over bloodied little Lebanon leads to serious hostilities over its partition between Israel and Syria, even so adept a statesman as Egyptian President Anway Sadat might find himself between the rock of his peace treaty with Israel and the hard place of standing alone against all other Arab states.

Evidence grows that young Egyptian army officers are pressuring Sadat hard for the American weapons promised as part of the Camp David accords. These weapons have been slow to arrive, a condition blamed by the restive younger generation on Israel's influence within the U.S. government.

Reagan courageously decided to meed that influence of Israel head on in the case of AWACS for Saudi Arabia. But the closer Israel comes to hostilities with Syria, the harder it is for Reagan to arm America's Arab friends if Israel objects. That gives Moscow a bonanza.

Soviet arming of Libya, for example, has reached alarming proportions: 475 aircraft, including 150 MiG-23s to Egypt's 200, the best of which are aging MiG-21s provided by the Soviets more than 10 years ago.

These military assets give Moscow impressive leverage, but as yet no diplomatic role in any Middle East settlement. The best chance the Soviets have had to wedge themselves into that role since the 1973 Yom Kippur war is right now, in the tragedy of Lebanon.