THERE ARE, UNFORTUNATELY, two brutal problems in Lebanon. One is the risk that the struggle there - better, the dozen or two overlapping struggles among different religious communities, classes, ideologies, private armies, individual power seekers and foreign states - will boil over into the Arab-Israeli dispute. Specifically, Syria, which is either trying to bring peace to Lebanon or trying to swallow the place, depending on your point of view, could collide with Israel, which is trying in its rough and confused fashion to keep Lebanon from becoming a Syrian-dominated "confrontation" state.

Precisely is prospect of an Israeli-Syrian confrontation, on the eve or in the midst of the Camp David summit, no less, prompted the surge of diplomacy whose results became at least partly visible yesterday. The Israelis, who have been sustaining Christian militiamen as a buffer along the Lebanese-Israeli border, finally agreed to admit United Nations peace-keepng forces, though not units of the pitifully small and weak Lebanese army, into the border zone.

The importance of this development lies in the fact that the Syrians and the Palestinians and the Lebanese government and the United Nations, for their separate reasons, simply could not abide a situation in which an Israeli-sponsored force rejected Lebanese and United Nations authority alike. Every day that those militiamen hung on, the tension grew. Yesterday's development does not end the danger. But it should quiet down the border as Egypt and Israel and the United States meet.

But the second brutal problem in Lebanon remains. It is the very real prospect - or so many Christians fear - that the Christian community, which has flourished in the open and pluralistic Lebanese society for centuries, will be institutionally and even physically destroyed by Moslem Arab forces led by Syria. New reports say that Syrian "peacekeeping" forces are currently killing Christian civillians by the hundreds north of Beirut. That Damascus can claim provocation does not alter the fact that, in upcountry Christian areas, the Syrian army is dominant and its victims are beyond the protection available to Christians near the Israeli frontier.

The United States is not insensitive to the peril of the Christians. Certainly to those Christians, however, and to a growing number of Americans, the administration appears ready to throw them to the wolves - the Syrians - for the sake of ensuring Syria's greater restraint in the Lebanese-Israeli border area and in respect to Mideast diplomacy as a whole. The Christians might be receiving greater international sympathy if they were a rare species in ecological danger, a senior statesman of the community, Charles Malek, observes. The point is that the Christians, for all that they have contributed to their own misfortune, are trapped, desperate and largely alone.

We don't have a foolproff prescription to cure sick Lebanon. But we do not hesitate to say that the United States must use whatever influence it has to press for a cease-fire, which would give the parties in Lebanon the room they need to sort out their own affairs free of foreign pressure. A preoccupation with the Arab-Israeli conflict cannot be allowed to distract the administration from a parallel concentration on the bloodletting in Lebanon.