In hindsight, the military forces of France, Italy, and the United States should not have left Lebanon. Their presence would most likely have prevented the Israeli occupation of west Beirut and the subsequent murderous events in the Palestinian refugee camps. But there is no reason why the multinational force should not be reconstituted and sent back to Beirut to displace the Israelis there. France and Italy say they are ready to rejoin a peacekeeping force. The United States should quickly and decisively add its support for such a move.
Some officials within the Reagan administration believe that the use of United Nations forces would be preferable and that some of the UN peacekeeping troops now stationed in southern Lebanon could be moved north. The idea seems dubious. The so-called UNIFIL contingents have not been able to prevent violence in the south because of the Israeli opposition to them. It is a question whether the Israelis and their Lebanese clients would allow UN units to do their job in west Beirut. On the other hand, perhaps it would be possible to recon-stitute the tripartite force and have it operate in coordination with a UN force. Bringing the UN in might be prudent from an international organizational point of view (although it has the added risk that the Soviets might get involved).
The American people would of course be taking a risk. They would have to accept that their troops might get caught in crossfire and become militarily engaged. There could be fighting and even casualties. But, against this potential hazard must be weighed the necessity of deterring further strife and killing in Lebanon, strengthening the Lebanese Army, helping it to take control, and providing stable conditions for establishment of a new Leba-nese government.
With his peace initiative of three weeks ago, President Reagan assumed leadership in the Middle East. His own diplomats worked out the agreements under which the PLO fighters were evacuated from Beirut and which guaranteed the safety of the Palestinian refugees. Now he has unequivocally ''demanded'' Israel's withdrawal from Beirut. If the United States fails to act to back up that demand it will lose its credibility and its ability to influence future events. It cannot hope to be a peacemaker in the Middle East unless it stops the deterioration of the Lebanese situation.
It may also have to consider other measures if the Begin government, which has rejected any responsibility for the massacre at Sabra and Shatila, refuses to pull out of Beirut and declines all efforts to negotiate a Palestinian settlement. A suspension of military and economic aid is no longer beyond contemplating. Short of any dramatic cutoff of such aid, Washington nonetheless could begin to signal its determination not to go on supporting Israeli policies that are harmful to US interests in the Middle East. Why, for instance, should the American people continue to finance Israel's colonization of the West Bank? It would not be unreasonable to withhold aid in the amount of the cost of any new settlements or expansion of existing ones.
Another American president, it might be recalled, turned to this form of economic sanction. In a move to get Israel out of the seized Sinai in 1956, Dwight Eisenhower gave instructions that the next administration aid request would contain nothing for Israel. His action succeeded. It was taken, moreover, on the eve of the November elections. Mr. Eisenhower faced considerable flak, to be sure, but his diplomacy worked.
That may have a lesson for today.