President Reagan's offer to guarantee Israel's northern borders once Israeli troops withdraw from Lebanon shows the depth of United States concern about moving forward the peace process in the Middle East. It is clear the President is agonizing over the continued presence of Israel in Lebanon and is hoping to break the negotiating impasse by risking having the US play a larger role.
The proposal raises many questions, of course. What does ''guarantee'' mean? Presumably it does not mean a bilateral security treaty with Israel, a major step which could only be imagined as a possible capstone to a comprehensive peace settlement. If it means providing American troops for a multinational force in southern Lebanon - and this would seem to be the minimum required of any ''guarantee'' - what happens if there are US casualties? What would be the force's mandate? Though presumably stationed there for intelligence and surveillance operations, would the troops be permitted to engage in fighting? What if PLO terrorists sneaked through and staged a hit-and-run attack on Israeli villagers? These are the unanswered questions which Congress is certain to look at closely and critically.
However uncomfortably the legislators might regard such a development, however, it seems evident that without strong American action of some sort any hope of persuading Israel to pull out of Lebanon will be virtually dissipated. The Israelis plainly want to keep their forces in southern Lebanon and also to use the militias of their Lebanese Christian ally, Major Haddad, to help secure the frontier. This would in effect undermine efforts to restore a united Lebanon and rebuild a national Lebanese Army capable of exerting authority throughout the country. But with a contingent of Americans and others manning listening posts in southern Lebanon and Arabic-speaking Lebanese Army units moving into the area - perhaps even with Major Haddad retaining some autonomy and role - the border region could be secured against the reinfiltration of PLO guerrillas.
Such an arrangement should in theory be acceptable to Israel. If it is not acceptable, it could only be concluded that Israel desires not security but land and that its primary objective in invading Lebanon was less one of cleaning out the PLO (who, after all, were largely abiding by the cease-fire) than of establishing a foothold that would give Israel permanent control of the Litani River - a dream that Ariel Sharon and other hard-line Israeli leaders have long had.
Mr. Reagan apparently realizes that the United States will have to pay some price in order to persuade Prime Minister Begin to withdraw from Lebanon and to begin negotiating the future of the occupied West Bank. But the President will need the American people behind him if he is to succeed, for in a preelection season he is bound to have a hard time persuading Congress to go along. Most legislators are not likely, in the midst of a presidential election campaign, to back US policies scorned by Israel. But if Mr. Reagan can take his case to the people - and perhaps his speech to the American Legion is a beginning - he might be able to overcome this political liability. If Mr. Reagan's goal is to further the interests of peace in the Middle East, including the long-term security of Israel - and certainly it is - this needs to be boldly explained to Americans at large as well as their lawmakers in Washington.
Mr. Reagan confronts his first critical test in the Middle East. Nothing will budge without the use of US power. On the mildly encouraging side, the PLO has given a ''yellow light'' to Yasser Arafat for a joint Palestinian-Jordanian delegation to start peace talks with Israel. But King Hussein is reluctant to take this chancy plunge into negotiation without evidence that Washington is prepared - and able - to influence Israel. So far the monarch has little to reassure him. Israel remains ensconced in Lebanon and continues to plant settlements in the West Bank, contrary to US wishes. With each passing day, the possibilities for President Reagan's peace plan thus seem to recede.
How determined is Mr. Reagan to sell his latest offer?