American Secretary of State George Shultz has begun to immerse himself in the Middle East. The most important aspect of his grueling shuttle mission is that he has been brought face to face with the complex realities of the Arab-Israeli confrontation and the difficulties of dealing with them. True, he has hammered out an agreement with Israel and Lebanon on terms for the withdrawal of Israeli forces from Lebanon. But whether this is more surface motion than substantive achievement is not yet clear.
The world will hope that Mr. Shultz has gained a bona fide breakthrough. His skills as a negotiator are admirable. Past experience should caution against premature euphoria, however. While Israel has accepted the accord ''in principle'' and is publicly striking a moderate posture, it is calling for ''clarifications'' on some aspects of the agreement. So there are still details to tie down. Will this take days? Weeks? Months? Meantime, the spotlight shifts to Damascus where Mr. Shultz tried unsuccessfully to persuade President Assad to accept the agreement.
Syria promises to be a tough nut to crack, and US diplomacy will be fully tested in the period ahead. The fundamental question is whether Syria really wants a withdrawal of all foreign forces from Lebanon and will simply exact the highest political price from the United States before agreeing to a pullout - or whether President Assad sees it in Syria's strategic interest to remain in the Bekaa Valley, even at the price of Israel's continuing presence in southern Lebanon.
It should not have surprised Mr. Shultz that the Syrians are not happy with the Lebanon-Israel accord. The agreement would permit an Israeli military presence in Lebanon, and Israel's ally, Major Haddad, would be in charge of antiterrorist intelligence and thus continue to operate as Israel's proxy. What especially rankles Syria is the false impression given that the presence of Syrian forces in Lebanon and the presence of Israeli forces are somehow linked and should be viewed alike. Indeed, the Syrians are not unjustified in pointing out that they are in Lebanon at the request of the Beirut government (and Arab League) while the Israelis violated international law by invading Lebanon. It seems clear that the US will not be able to conclude any agreement with Syria for a troop pullout which does not take account of this difference.
The next challenge for Mr. Shultz (in addition to those ''clarifications'') will be to persuade President Assad that Syria's long-range interest does lie in a simultaneous withdrawal of foreign forces from Lebanon. The risks of not moving in that direction, of solidifying a de facto partition of Lebanon, are self-evident: continued chaos in Lebanon and the potential of another Israeli-Syrian conflict, escalating perhaps into a confrontation between the US and the Soviet Union. Surely that is not something Syria wants. The dividends, on the other hand, could be appreciable: the withdrawal of Israel from an Arab land and movement on the larger issue of a comprehensive peace settlement.
It is on the latter score that Washington can make its influence felt. Unfortunately, the US has not paid sufficient attention to Syria - injudiciously leaving the field to the Soviet Union - and now it must make up for lost time. The Reagan administration is on record as saying that the President's peace plan for the West Bank includes US support for a return to Syria of the Golan Heights , a chunk of territory which Israel has peremptorily annexed. But Mr. Assad may be looking for a firm US commitment to regain the Heights before he agrees to pull out of the Bekaa Valley. Washington should be willing to assure him on this matter, and to persuade him that Syria's long-term interests lie not with Moscow but in closer ties with the West.
Regardless of US policy in the past, President Assad should see that opportunities now exist for diplomatic progress. At the moment Israel has in effect captured the public limelight by assenting to the compromise agreement worked out by Secretary Shultz. This apparent Israeli reasonableness is certain to gain the upper hand with American public opinion if the Arabs are seen to be the ones now dragging their feet. The Palestine Liberation Organization already is losing ground among many Americans for not going to the negotiating table. If Arab leaders are unwilling to swallow their pride and take a risk for peace, they cannot blame the US for the impasse in the Middle East.