THE AMERICAN INITIATIVE to replace the Israeli troops in southern Lebanon with a U.N. peacekeeping force is a natural. The Lebanese government, rendered irresponsible by weakness, has been unable to police Palestinian guerillas there. Syria, whose troops keep a shaky truce in Lebanon's civil war elsewhere in the country, has been unwilling to take on border policing. Israelis had hoped that their support of Christian villagers in southern Lebanon would seal the border, but the bloody Palestinian raid of last Saturday demonstrated spectacularly the inadequacy of the seal.

The Israeli drive into southern Lebanon has solved a particular security problem but not for long. Maintaining the occupation will only add further to a toll of civilian casualties that is excessive even by the license given a nation acting as Israel was in sore provocation and in self-defense against Palestinian terrorists. Continued occupation will ensure more Israeli-Palestinian battling, this time on a new line, make it harder for Egype to resume peace talks with Israel, and crank a new and distracting source of misunderstanding into Israel's relations with the United States.

Enter the United Nations. Peacekeeping is one activity in the Mideast that, since 1973, the world organization has learned to do well. Its units are already in place on the lines between Israel and, respectively, Egypt and Syria. On account of the particularly volatile circumstances in Lebanon, appropriate rules of engagement will have to be advised. Israel, as a U.N. member, would be honor bound to respect a U.N. peacekeeping force, and it would have every incentive to do so as long as the force performed its mission effectively. To be sure, it is bizarre not to say revolting to see the PLO, self-proclaimed author of the recent attack in Israel, taking part in the current Security Council session on the Lebanese question. But it is hard to imagine the PLO, which prizes its status at the United Nations, being so stupid or arrogant as to attack U.N. peacekeepers. The Syrians, who could do nothing to stop the Israeli invasion of southern Lebanon, may huff and puff but they will probably find that a U.N. force is for them a facesaver and a guard against collision with the Israelis in Lebanon. As go the Syrians on this question, so presumably go the Russians.

There is yet another reason for moving quickly to put a U.N. force into place, and that is to keep the Lebanese question from clogging the American-Israeli agenda at a moment when President Carter hopes to engage Prime Minister Menachem Begin in discussions aimed at resuming peace negotiations between Egypt and Israel. Mr. Begin is due in Washington on Tuesday, and there are signs that he might prefer to give Lebanon priority over the more central and vexing issues that Mr. Carter wishes to get cracking on. For Mr. Carter to move with all deliberate speed, however, he must show Israelis that he is acting in Lebanon in a way that respects their legitimate demands for adequate temporary security on their norther frontier, pending a peace agreement.

This is precisely the bill that a U.N. peace force could fill. Establishment of such a force would demonstrate that the international community, and the United States in particular, do understand Israel's virtually unique situation in having a neighbor unable or unwilling to hald continuous murderous attacks across the border. And in demonstrating that careful international arrangements can provide an effective substitute for the occupation of Arab land, the establishment of a new U.N. force could also serve as a useful model for further negotiated steps on Israel's other frontiers.