A SEQUENCE STARKLY revealing of the true source of Lebanon's anquish emerged from the Arab League summit in Tunis. The Lebanese president, Elias Sarkis, demanded that Palestinian guerrillas withdraw from southern Lebanon and stop launching attacks on Israel, so that Lebanese sovereignty could be asserted there and so that Israel would have no further legitimate cause for strikes into Lebanon. For his pains, however, Mr. Sarkis was reviled by PLO leader Yassar Arafat and rebuffed by the other Arab states. To salve his wounds, the Arab League offered Mr. Sarkis instead a promise of $2 billion over five years, half of it intended for reconstruction in the battered south.

To be sure, things are never quite what they seem in Lebanon. It is altogether possible that behind the veil of defiance drawn by the Arab summit, the PLO is being leashed. This may be coming about not by any decision taken in Tunis but by events leading up to the summit. Israel's strikes into Lebanon and perhaps stirred second thoughts in Syria, which is ill prepared (now that Egypt has made peace with Israel) for mounting hostillites in Lebanon and which is, anyway, in the toils of a deep domestic crisis. The PLO, by continuing to use southern Lebanon as a launching pad, was souring its position at the United Nations, which provides peacekeepers in southern Lebanon and which also provides the principal forum in which the PLO conducts it diplomacy against Israel. Meanwhile, the Israelis, by quietly reducing the ferocity and open-endedness of their attacks, were making it easier for the other Arabs to restrain the PLO.

In any event, it is good to see an authoritative Arab scotching of the familiar argument, so often trotted out by the State Department, that the essence of the problems in southern Lebanon is a "cycly" of violence in which Palestinian and Israeli roles are equal and indistinguishable. Mr. Sarkis, who knows better, put his finger on the single cause: the PLO. Evidently, Mr. Arafat contended merely that the PLO has a "right" to fight where it can. The other Arab states (excluding Egypt, which wasn't invited to the summit) showed the disrespect in which they hold helpless Lebanon's sovereignty by formally endorsing the PLO view. In fact, until they unequivocally reject that view, Lebanon can look forward to no more than alternating phases of devasting war and uncertain truce.