WHAT ABOUT Syria's rejection of the American-mediated agreement between Lebanon and Israel? It's no academic matter. Israel's withdrawal from Lebanon is conditioned on Syria's parallel withdrawal. A Syrian veto could condemn hapless Lebanon to continued foreign violation. It could lead on to an Israeli-Syrian war. There seem to be two broad views of what the Syrians are up to. Fortunately, Secretary of State George Shultz is taking the second.

The first view is that for assorted considerations of intra-Arab rivalry, Soviet manipulation and Syrian intrigue, Damascus is determined not to quit Lebanon, even if that costs it the opportunity to negotiate on other vital Syrian interests (Golan and the Palestinian issue) and even if this course increases the risk of war with Israel. It would be embarrassing for President Hafez Assad to go back on his pledge to respect a Lebanese request to withdraw--such a request has not yet been formally made. But the grid of pressures, foreign- and self- generated, in which he is locked may keep him on his current course.

The second view holds that Syria has not so much vetoed the price that Lebanon is prepared to pay for Israeli withdrawal as it has demanded to be compensated analogously for its own withdrawal. President Assad's nose is out of joint: he cannot bear that Israel should receive more in political and security advantage for invading Lebanon than Syria should receive for having been invited in and performed a thankless peace-keeping task for seven years. Syria does not want a war with Israel, whose guns sit within range of Damascus; its current huffing and puffing is meant to convey that it would be entering negotiations from a position of strength.

There is no guarantee that negotiations will pay off. But why yield prematurely to a fatalistic reading that prevents the United States from making a try? At the end of the road, at best, lies a Lebanon with an Israeli- influenced sphere in the south and a Syrian-influenced sphere in the east. The Lebanese understand that's a lot better than the condition they are enduring now. It would be "naive of us to accept the presence of 50,000 Israeli occupation troops in order to refuse 50 Israeli observers as part of an international supervisions committee," Lebanese President Amin Gemayel said yesterday, insisting that his country would not bow to "Arab blackmail."

Give Lebanon a chance, George Shultz said in London. That's the main point.