The U.S. Multinational Force (USMNF) operated in Beirut, Lebanon from 25 August 1982 to 26 February 1984. During this period four different MAUs served as peacekeepers. The terrorist bombing of the US Marines barracks became a quintessential exemplar of the conditions under which military intervention may not be effective.

Israeli-Palestinian fighting in July 1981 was ended by a cease-fire arranged by U.S. President Ronald Reagan's special envoy, Philip C. Habib, and announced on July 24, 1981. The cease-fire was respected during the next 10 months, but a string of incidents, including PLO rocket attacks on northern Israel, led to the 06 June 1982, Israeli ground attack into Lebanon to remove PLO forces. Israeli forces moved quickly through south Lebanon, encircling west Beirut by mid-June and beginning a three-month siege of Palestinian and Syrian forces in the city.

Throughout this period, which saw heavy Israeli air, naval, and artillery bombardments of west Beirut, Ambassador Habib worked to arrange a settlement. In August 1982, he was successful in bringing about an agreement for the evacuation of Syrian troops and PLO fighters from Beirut. The agreement also provided for the deployment of a three-nation Multinational Force (MNF) during the period of the evacuation, and by late August 1982, U.S. Marines, as well as French and Italian units, had arrived in Beirut. On 10 August 1982 the alert posture of the Mediterranean Amphibious Ready Group was heightened in light of a likely deployment as part of a peacekeeping force to oversee the evacuation of Palestine Liberation Organization (PLO) forces from West Beirut.

The 32d Marine Amphibious Unit (MAU) from Camp Lejeune deployed to Beirut to oversee the safe departure of thousands of Palestine Liberation Organization (PLO) fighters out of the war-torn city. On 24 August (EDP), the first of 800 Marines began going ashore at Beirut as part of a joint U.S.-French peacekeeping force. When the evacuation ended, these units departed. On 8 September, following the removal of the PLO forces from West Beirut, the Marines redeployed aboard the MARG ships. The US Marines left on 10 September 1982.

In spite of the invasion, the Lebanese political process continued to function, and Bashir Gemayel was elected President in August, succeeding Elias Sarkis. On September 14, however, Bashir Gemayel was assassinated. On 15 September 1982, Israeli troops entered west Beirut. During the next three days, Lebanese militiamen massacred hundreds of Palestinian civilians in the Sabra and Shatila refugee camps in west Beirut. Bashir Gemayel's brother, Amine, was elected President by a unanimous vote of the parliament. He took office 23 September 1982.

MNF forces returned to Beirut at the end of September 1982 as a symbol of support for the government. On 22 September 1982, following the Phalangist Christian force massacre of Palestinians in the Sabra and Shatilla refugee camps, the Mediterranean Amphibious ready Group was ordered to the Eastern Mediterranean. President Ronald Reagan ordered the 32d MAU back into Lebanon to support the Lebanese Armed Forces where it was soon relieved by Camp Lejeune's 24th MAU. The 1st Battalion, 8th Marines Headquarters building was located at the Beirut International Airport and housed the Battalion Landing Team (BLT). From 27 September through 21 January 1983, two carriers were tethered to Lebanon to provide support for the Marine Corps forces ashore. On 11 February 1983, the response posture for carrier support was relaxed as the situation had stabilized. In February 1983, a small British contingent joined the U.S., French, and Italian MNF troops in Beirut.

On 17 May 1983, an agreement was signed by the representatives of Lebanon, Israel, and the United States that provided for Israeli withdrawal. Syria declined to discuss the withdrawal of its troops, effectively stalemating further progress.

The USMNF was initially successful; but, as the strategic and tactical situations changed, the peacekeepers came increasingly under fire. Opposition to the negotiations and to US support for the Gemayel regime led to a series of terrorist attacks in 1983 and 1984 on US interests, including the bombing on 18 April 1983 of the US embassy in west Beirut (63 dead), and of the US embassy annex in east Beirut on 20 September 1984 (8 killed).

Just before 6:30 a.m. on Oct. 23, 1983, a Mercedes truck passed a Lebanese checkpoint on the airport road without halting. The truck turned into the airport parking lot, circled twice and picked up speed for a deadly run at the headquarters building. Orders prohibited Marines from being locked and loaded, but small arms fire probably would not have made much difference, according to reports. A sentry did get some shots off with a pistol, however. The driver of the speeding van was determined to put a huge dent in the American presence in Lebanon. After breaking through several barriers, it sped between two sentry boxes and crashed through more obstacles, penetrating the building's first floor before detonating tons of explosives, taking the lives of 241 Marines, Sailors and soldiers, a majority of which were stationed at Camp Lejeune. Most died in their sleep or were crushed as the building collapsed, while a handful have died in the years that followed due to injuries sustained from the bombing.

On 3 December 1983, two F-14s flying over Lebanon were fired upon by Syrian antiaircraft artillery. On 4 December 1983, aircraft from Kennedy and Independence were launched against Syrian targets; two were shot down, and one U.S. airman was taken prisoner by Syrian troops.

The virtual collapse of the Lebanese army in February 1984, following the defection of many of its Muslim and Druze units to opposition militias, was a major blow to the government. As it became clear that the departure of the US Marines was imminent, the Gemayel Government came under increasing pressure from Syria and its Muslim Lebanese allies to abandon the May 17 accord. On 26 February 1984, the withdrawal of the USMC contingent of the international peacekeeping force was completed. The Lebanese Government announced on 05 March 1984 that it was canceling its unimplemented agreement with Israel.