SOMETHING STUNNING may be happening in Lebanon, which has suffered enough to deserve it. In the first instance, the threat of a war there between Israel and Syria has substantially receded. In the second, the outlines of a process conceivably leading to a reconciliation of the long-warring factions within the country are coming into view. If it all sounds tentative and uncertain, it is. It's promising, too.

The big new event is the peaceful breaking of the Syrian siege of Zahle. This is the Christian town in eastern Lebanon that, three months ago, bid to become the spark of a major conflict. The other day the Arab League successfully arranged for the defending Phalangist militiamen to be replaced by Lebanese government security forces. This lets the Christians claim they saved the city and the Syrians claim they nipped an Israeli-backed Christian power play. It clears the way for Syria's removal of the missiles it emplaced to protect its besieging forces, and for Israel's lifting of its threat to knock out those missiles. It establishes a formula -- replacing private foreign-connected armies with official Lebanese forces -- that can perhaps be extended to divided Beirut now and to other danger zones later. It starts to lower the strictly military obstacles to a fresh attempt by the Christian and Moslem communities to reconstitute a united Lebanon.

No one could have predicted three months ago that the crisis would take this turn. Unquestionably, the prime credit must go to the Arab mediators, especially Saudi Arabia and Kuwait. They managed to break out of their customary leave-it-to-Syria detachment from Lebanon and to take the political risk of attempting to set up a new, broader Arab framework. The sense of fatalism verging on indifference that has often and lamentably characterized the Arab attitude toward Lebanon seems to have been broken, at least temporarily. a

The Reagan administration has, after a rough start, played the crisis with finesse. It used its standing in Israel, and perhaps the extra claim on Israeli attention it gained after the Israeli raid in Iraq, to persuade Menachem Begin not to preempt diplomacy by prematurely attacking the Syrian missiles. Ambassador Philip Habib shuttled skillfully around the area, leaving it properly unclear whether he was deftly putting hands on or just as deftly taking hands off. In the time thus bought, the Arab League did the work in which, fortunately, it is still engaged.