In addition to the background of the overall U.S.-Israeli relationship, the principal accomplishment of the multinational force was to facilitate the exit of the PLO from Lebanon, which was part of Israel’s objective in invading Lebanon in the first place.
Besides, as with other deployments of U.S. military personnel to already dangerous places, the lethal logic of force protection kicked in, and peacekeeping morphed into offensive action. President Reagan authorized “aggressive self-defense” against hostile forces that posed a threat to the Marines, with those same hostile forces also being adversaries of Israel and its Lebanese militia allies. The U.S. engagement on the ground was supported by naval gunfire, which would later include the battleship New Jersey firing 16-inch shells at targets in the mountains near Beirut.
All this was in addition to the usual loathing by domestic elements of any foreign military presence, which at other times and places has fed violent responses, including suicide terrorism.
The deadliest response to what the United States was doing in Lebanon came on October 23, 1983, when a suicide driver drove an explosives-laden truck into the building at the Beirut airport the Marines used as a barracks.
Read it in full here: https://responsiblestatecraft.org/marine-barracks-bombing-in-beirut/
best
Kelley