ISRAEL AND LEBANON are to open talks Monday on the terms for withdrawal of the 12,000-14,000 troops left over from the Israeli invasion of Lebanon in 1982. That's good news. Success would restore to Lebanon a piece of its sovereignty; for Israel it would be a useful deliverance. But the question of the Syrian attitude -- the question that spoiled the last withdrawal talks -- remains open. The Israelis, necessarily, demand guarantees for the security of their northern border; given the weakness of the Lebanese government and army, these guarantees can come only from Damascus. But the Syrians publicly reject giving such guarantees. There, uneasily, the matter rests.

The immediate sponsor of Monday's talks is the United Nations, much cursed and much needed. It disposes of nearly 6,000 peacekeepers in southern Lebanon. Ieir number can be increased, their turf expanded and their authority bolstered, they can provide much of the necessary buffer between Israelis and assorted Lebanese (and Palestinians) along the coast, and between Israelis and Syrians in the Bekaa valley.

That still leaves the Israeli-Syrian impasse, complicated as always by Syria's refusal to talk face to face. There are two ways to break it. One is by having a war. Syria, seeing Israel writhing under the pressure of its Lebanon casualties, may elect to sit tight, and Israel may then find an occasion to turn up the military heat. This would be an appallingly reckless course on the part of both, but it could happen, and soon.

Could American diplomacy help bridge the Israeli-Syrian gap? The Reagan administration, though aware of the volatility of the situation, is plainly reluctant to commit more of its prestige to the Lebanese enterprise. Its readiness to shoulder new political risks may grow, however, if it wins reelection on Nov. 6. The diplomatic terrain, though rough and bumpy, is not altogether unfamiliar or unpromising: it was Americans who arranged the Syrian-Israeli disengagement that has worked flawlessly on the Golan Heights for nearly 10 years. The requirement now is for more damage control -- not a grand mission but a necessary one.

Something important tends to get left out of discussions of Lebanon: Lebanon. Currently the danger and therefore the political drama lies between Syria and Israel. But both Syrians and Israelis will manage to care for their interests, even without a new arrangement in Lebanon. The Lebanese cannot care for their security or even for their territorial integrity. The United States has a commitment to Israel and a certain connection with Syria. It also has a moral obligation to do what it can for Lebanon. We recognize the difficulties of prescription here. Yet the obligation must be served.