The next phase of American Mideast involvement will be focused on how to keep Syrian opposition from sinking the draft agreement on Israeli troop withdrawals from Lebanon.
United States Secretary of State George Shultz has made clear that despite the objection of Syrian President Hafez Assad, whom he met for four hours on Saturday, the agreement painstakingly worked out between Israel and Lebanon will not be renegotiated.
Efforts are being made to wrap up clarifications requested by both Israel and Lebanon before the agreement can be formally signed. But US officials must also find a way to get Mr. Assad to accept the terms of the draft agreement and to withdraw Syrian troops from Lebanon, or the hard-won documents will become just another piece of paper.
For without withdrawal of Syrian troops along with the forces of their allies in the Palestine Liberation Organization, Israeli troops, too, will stay in Lebanon. Most probably Israel would withdraw to 28 miles north of its border, creating a de facto partition of Lebanon with Israel in the south and Syrian and Palestinian troops in the north and east. Israeli pessimism was reflected in Israeli Defense Minister Moshe Arens's remarks Sunday that he was ''not overly optimistic'' that Syria would withdraw its troops.
Mr. Shultz apparently believes he has a chance to reach an agreement, despite President Assad's harsh statements. Shultz's hopes appear to be based largely on encouragement received from Saudi Arabian leaders during his visit there. The Saudis reportedly told Shultz that the Syrian President, who arrived in the country just after Shultz left, had promised to withdraw his forces from Lebanon at the same time as the Israelis pulled out.
The secretary of state also said that the one hopeful sign in his Damascus talks was that Assad had told him ''the door is open for further talks,'' leaving open the possibility that US envoy Philip C. Habib, if not Shultz himself, could follow up contacts with the Syrians.
The harsh Syrian reaction could not have come as a surprise. Assad is known as one of the Mideast's toughest bargainers, a lesson former Secretary of State Henry Kissinger learned when he conducted disengagement negotiations between Israel and Syria over the Golan Heights in 1974.
In the second volume of his memoirs, ''Years of Upheaval,'' Dr. Kissinger recalls how Assad let him think at the conclusion of a month of negotiations that they had failed. Just as Kissinger prepared to leave, in final acceptance of defeat, the Syrian President conceded. ''It was a stunning performance,'' Kissinger wrote. ''Assad had played out the string to absolutely the last possible millimeter. . . .''
Over the last eight months the Syrians have consistently taken a tough line on what would be acceptable terms of an Israel-Lebanon agreement. They have insisted that any residual Israeli troop presence in south Lebanon or normalization agreement with Lebanon would threaten their security.
The draft agreement permits only a token force of 50 Israeli soldiers to operate in Lebanon under command of Lebanese officers in joint supervisory teams , and it waters down the role of Israel's Lebanese ally, militia leader Maj. Saad Haddad. But Syria has chosen to take this as an indication that Israel will maintain an indirect presence in Lebanon.
Nor are the Syrians in any hurry to see a troop withdrawal implemented. The longer the proceedings draw out, the more attention the Arab world and the US must pay to Syria's positions; the harsher is the reminder to Lebanon of their big neighbor's imposing presence; and the higher the casualty toll of Israeli soldiers remaining in Lebanon. Daily shelling between pro- and anti-Syrian factions is expected to continue as long as the matter remains unsettled.
Perhaps in recognition of these factors, reports from Washington before Shultz left on his Mideast mission indicated he believed the Syrian phase of the negotiations could take as long as four months. Highly placed sources in Jerusalem say Israel will not set a time limit for these negotiations.
The key unanswered question is whether Assad, despite his harsh words, intends to leave Lebanon after exacting the best possible bargain or whether he can be persuaded by pressure or self-interest to change his tune.
The American strategy appears to be to let others apply the pressure. Officially the next ''Syrian'' phase of the process is being described as one of ''very difficult negotiations'' between the Syrians and the Lebanese.
In reality, the US hope is that the moderate Arab states, especially Saudi Arabia, will hold Assad to his promise made at the Arab summit in Fez, Morocco, to quit Lebanon if Israel did, or brand him with the onus for a continued Israeli troop presence in Lebanon. Presumably a commitment to leave Lebanon would be sweetened by the promise of additional Arab financial aid.
US officials say the very real threat of a war between Syria and Israel, whose troops in Lebanon's Bekaa Valley sit within some 15 miles of Damascus, must make Assad ponder whether he would again be willing to risk almost certain defeat.
But the Saudi carrot and the stick of imminent war may not be sufficient to convince Assad. The US earlier overestimated the ability of the Saudi regime to pressure the PLO into giving Jordan's King Hussein the green light to enter peace talks with Israel, and the desert rulers appear more comfortable when organizing Arab consensus than when chastising dissent. Moreover, informed analysts in Damascus doubt whether money or even the threat of war will sway the Syrian regime. The possibility of war is seen in Damascus as less ominous now that Soviet military backing has given the Syrians more confidence that they could at least give Israel a tough fight.
The US may find that the Syrians are holding out for more American concessions before making a deal. There has been speculation that the Syrians want a US commitment to help win back the Israeli-occupied Golan Heights, captured from Syria in 1967 and annexed in de facto fashion by Israel in 1981.
But highly placed Syrian officials insist that the Golan Heights are not their key objective. They play down a much-publicized recent letter from President Reagan repeating the longstanding American position that United Nations Resolution 242, which calls for return of Israeli-occupied territory for peace and is the basis for all recent US peace proposals, also applies to the Golan Heights.
''We always say the Golan is secondary,'' insisted a high-ranking official of the ruling Syrian Baath Socialist Party in Damascus recently, while taking a swipe at the credibility of US promises. He insisted that in the Syrian view the return of the Golan Heights must be linked with the question of the Israeli-occupied West Bank and Gaza Strip and the right of the Palestinians to a state.
In fact, a key question affecting US prospects for achieving a Syrian agreement will be whether the Assad regime insists on linking such an agreement to a wider regional peace plan involving the participation of their Soviet allies. But American officials acknowledge, given the political tendency of the Reagan administration and the approach of a US election year, that such an American concession would be almost impossible to make.