With American credibility throughout the Middle East at stake, President Reagan has decided to make a new plunge into Lebanon peacekeeping.
Officials say the President still pins a great deal of hope on the Lebanese themselves, and their well-known resilience. But in the wake of the assassination of President-elect Bashir Gemayel, the Israeli drive into west Beirut, and now the massacre of unarmed Palestinian refugees, the Lebanese government issued a new call for limited, temporary international help. At press time, it appeared that President Reagan decided to respond by sending a contingent of US marines back into the beleaguered city.
Despite the obvious dangers involved, President Reagan decided to throw his support behind proposals for a new international peacekeeping role in Lebanon apparently because of the equally great dangers of US inaction. Not the least of those dangers is a complete loss of American credibility among the Arab nations should the administration fail to act. The Reagan administration had given its guarantee that innocent noncombatants would be protected once the forces of the Palestine Liberation Organization were withdrawn from Beirut, but such protection was lacking.
No one denies that US involvement in a new Beirut peacekeeping effort would entail the risk that US soldiers might be endangered. The US Defense Department has been most reluctant to commit itself to playing any role in what one Pentagon official called the ''Lebanon morass.'' But the risks were seen here as considerably reduced by the fact that the Lebanese themselves, despite multiple crises, seemed to be pulling together in an orderly fashion:
* The Lebanese have called for an election on Sept. 21, and most observers were predicting that Amin Gemayel, brother of the slain President-elect, will easily win the vote, thanks in part to heavy support from the Muslim community.
* Units of the Lebanese government army were reported to be taking over some positions in west Beirut from Israeli units, a sign that the Israelis may be willing to make gestures in order to redeem their tarnished image.
''The Israelis have been turning various positions over to the Lebanese army . . . ,'' said Ghasan Tueni, Israel's ambassador to the United Nations. ''Twenty-five UN observers have already gone to Beirut. So there is movement.''