President Reagan’s offer “in principle” to contribute ”a small contingent” of combat troops to an international force for evacuating Palestinian guerrillas from Beirut remains academic for the moment. However, the situation could change quickly, and five U. S. warships have sailed within 150 miles of the Lebanese coast ready to land up to 1,800 Marines if so ordered. Reagan’s offer was subject to “certain conditions,” including a formal request by the Lebanese government and acceptance by “all parties,” neither of which had been forthcoming as this was written. The PLO said a flat no to a U. S. evacuation mission, but tempered this if the force is international, for example under sponsorship of the United Nations. The specific mission which sparked cautious reaction pro and con by members of Congress would be to aid the Lebanese armed forces in overseeing the orderly departure of armed personnel and assist Lebanon’s government in the transition. The guerrillas’ destination, say Egyptian sources, would be various Arab countries. What conditions the President specified were not immediately disclosed other than the formal Lebanese request and acceptance by all parties. Certainly, as both Sen. Charles Percy, chairman of the Senate Foreign Relations Committee, and Senate Democratic leader Robert Byrd pointed out, the U. S. must not act alone in sending troops. Moreover, any American involvement should await a formal settlement in Lebanon that would guarantee peaceful withdrawal of the Israelis as well as the Palestinian guerrillas from that beleaguered country. (Repeated ceasefire violations should underline the need for wariness.) A time limit – say 30 days – for the operation should be another requirement plus a stipulation that U. S. troops would not be involved in occupying Lebanon on a peacekeeping assignment. Already, with the PLO seemingly on the way out, the seeds of a new Civil War between the Christians and Moslems reportedly are taking root in the rubble of the Israeli invasion that trapped the Palestinians in a ring of armor. Evacuation of the guerrillas and departure of the Israelis would be a humanitarian act of monumental proportions. But to commit our troops to any participation beyond the evacuation would be a mistake. The scenario is reminiscent of 1958 when President Eisenhower responded to Lebanon’s request by sending Marines ashore in the Beirut area to prevent collapse of the Lebanese government in a Moslem – Christian war. Americans will be watching with concern as events in the present crisis unfold. The end of the Israeli – Palestinian presence in Beirut can’t come too soon, but full agreement on this should be a precondition to an intemational evacuation effort. It is hoped that any U. S. troop participation will proceed only with the considered judgment of the President and commander – in – chief, backed by a consensus of congressional leadership.