The signing of the US-mediated troop withdrawal accord between Israel and Lebanon has produced a tangible foreign policy success for the Reagan Adminstration.
But from now on Mideast eyes will be on the Americans to see if they can find the key to overcoming Syrian opposition, which would prevent the pact from ever taking effect.
United States strategy for winning over the Syrians has centered on hopes that Arab moderates, notably Saudi Arabia, would persuade the Syrians to support the agreement. Without Syria's agreement to pull back its 40,000 troops from Lebanon, Israel will not be obligated to withdraw its troops.
''We are (banking) on the moderate Arabs to feel that the alternative of chaos in the Middle East is just too threatening,'' a US official said.
Initially, US Secretary of State Shultz emphasized that the next phase in Lebanon, following the signing of the accord, should be mediation between Lebanon and Syria.
The US has also banked on the Arab states to ultimately persuade Syria and the Palestine Liberation Organization to remove their veto on the wider Reagan Mideast peace plan, which calls for King Hussein of Jordan along with non-PLO Palestinians to negotiate with Israel over the future of the Israeli-occupied West Bank. This strategy is still operative even though the Saudis have failed thus far to dissuade the PLO from vetoing the King's participation in the Reagan plan.
The US can take satisfaction not only from brokering the Lebanon accord, but also from the active or at least passive support it has received from the PLO and all Arab countries except Syria, South Yemen, and Libya.
And the Americans cannot be displeased at the troubles within the PLO. Yasser Arafat, chairman of the PLO, has been facing dissent inside his Fatah faction in Lebanon's Bekaa Valley, apparently over the appointment of two commanders who are accused by many of the rank and file of not having stood and fought during the Lebanon war.
Despite US expectations at this point, the moderate Arab states - having given Lebanon the green light to sign the accord - now appear to be waiting for the US to provide the mechanism that would lead to the implementation of the accord. Lebanese President Amin Gemayel, in particular, has made clear his country's dependence on the US to persuade Syria to go along.
Moreover, the Saudis, always uncomfortable playing the role of the ''heavy,'' have failed in the past to convince Syria to pull out of Lebanon and have ambivalent attitudes toward the Syrian regime. Concerned about Iranian attempts to stir up the minority Shiite sect's rebellion inside their kingdom, the Saudi rulers must consider whether they might one day need the intervention of Syria, which is the only Arab country to maintain close ties with Khomeini. Moreover, according to informed Arab sources, the Saudis are also ambivalent about US ability to convince Israel to accept a regional solution amenable to the Arabs. These doubts are reflected in their apparent neutrality on the Lebanon issue.
Mr. Shultz and other senior US officials have made it clear in recent days that the US is prepared to get involved in a Lebanese-Syrian mediation effort to find terms acceptable to the Syrians. In extremely conciliatory language, Mr. Shultz has also sent out strong signals to the Syrians that American-Syrian ties could be improved by Syrian cooperation in the troop withdrawal arrangement.
But senior US officials admit in private that their leverage on Syria is minimal. State Department spokesman Alan Romberg on Monday shot down a much-discussed potential quid pro quom for Syrian concessions: involvement of the Soviet Union in a Mideast peace conference with a broader mandate than the Reagan plan framework, which would include Syria and the PLO among the participants.
Romberg said the US sees no role for the Soviet Union in current negotiations over troop withdrawal nor does the US feel there is a Soviet role in the broader peace process.
Both the moderate Arabs and the US administration are likely to continue to hope that the other will instigate movement toward peace in Lebanon and in the region. But some US officials hint that should the moderate Arabs fail to pressure Syria to accept a Lebanon accord, they may have to bear responsibility, as the US enters an election year, for suspension of the process. ''The Arabs have to realize that Reagan has had his success in the Lebanon accord,'' said one source. ''He can just stop here and leave them in a mess.''