IT WOULD BE foolish to take for granted that the United Nations Security Council will automatically keep renewing the mandate of its peace-keeping force in southern Lebanon, as it did for five months in a vote the other day. UNIFIL, as the U.N. bandage in Lebanon is called, has a thankless job.It is trying to restore the authority of the Lebanese government, which militarily speaking is little more than a handwringing operation, in a tense border region where its authority is evaded or challenged by 1) Palestinians bent on attacking targets in Israel, 2) Christian militiamen sponsored by Israel, and 3) Israelis. The force for five months rather than the expected six is thoroughly understandable.

The Palestinians, aware that they owe much to the United Nations, have tried to run their raids in ways that would avoid openly flouting UNIFIL's authority. Not so the militiamen, who, with Israeli connivance, have blatantly prevented the United Nations from buckling the border belt up tight. The militiamen have their own reasons. Israelis' explanation for supporting them is that UNIFIL cannot render the border totally impermeable -- a contention ignoring the twin facts that the Israelis have not given UNIFIL a fair chance and that Israeli forces did a lot worse before the international force arrived on the scene.

The Security Council, in renewing the UNIFIL mandate, failed one duty by not criticizing Palestinian guerrillas for launching raids on Israel in the first instance. Nonetheless, Israel deserved the criticism it received for failing to cooperate with U.N. force. It leaves itself in the exposed position of asking understanding from the very international community whose peace-keeping force, installed for its as well as others' benefit, it is frustrating.

The basic problem in Leganon, to be sure, is that civil war destroyed the authority of the Lebanese government, creating the southern Lebanon vacuum entered in turn by marauding Palestinians, invading Israelis and international peace-keepers. The basic answer is to put Lebanon back together again, but that is an excruciatingly difficult job involving not only the conciliation of the Lebanese but a solution of the Palestinian question. In the interim -- unavoidably a long interim -- the United Nations is the body most fit to keep border tension from exploding into major hostilities and, not incidentally, to minimize further misery among the simple people who live in the region. It is a bandage, but no less vital for being so.