The Marines had been on a peace-keeping mission in Beirut. The mission started going awry when the Reagan administration abandoned its neutral approach in the civil war then ravaging Lebanon and took sides—propping up the Christian government of Amin Gemayel and bombing Shiite and Druze areas of Lebanon. Shiites, at first welcoming of the American mission, turned. Marines began taking fire from Shiite militants—and responding in kind.
In April 1983, the American embassy in Beirut was bombed, killing 63, including 17 Americans. The attack on Marine headquarters, which coincided almost to the minute with a similar attack on French peacekeepers’ barracks elsewhere in Beirut, was not entirely unexpected. The multinational force (including Italians, who were not targeted by the twin attacks) knew by then that it was unwelcome by Shiites in Lebanon. Only the ferocity of the October bombings took the French and Americans by surprise. Within months, the multinational force had left Beirut.
Hezbollah’s Imad Mugniyah is believed to have masterminded both bombings.
For Hezbollah, 1983 was only a coming out. The organization’s fighters had been trained by the Palestinian Liberation organization in South Lebanon before the 1982 Israeli invasion drove out the PLO. Hezbollah was also (and still is) trained and funded in large part by Iran, and to a smaller extent by Syria.