On October 23, 1983, two truck bombs were detonated at buildings in Beirut, Lebanon, housing American and French service members of the Multinational Force in Lebanon (MNF), a military peacekeeping operation during the Lebanese Civil War. The attack killed 307 people: 241 U.S. and 58 French military personnel, six civilians and two of the attackers.

Early that Sunday morning, the first suicide bomber detonated a truck bomb at the building serving as a barracks for the 1st Battalion 8th Marines (Battalion Landing Team – BLT 1/8) of the 2nd Marine Division, killing 220 Marines, 18 sailors and three soldiers, making this incident the deadliest single-day death toll for the United States Marine Corps since the Battle of Iwo Jima in World War II and the deadliest single-day death toll for the United States Armed Forces since the first day of the Tet Offensive in the Vietnam War. Another 128 Americans were wounded in the blast. Thirteen later died of their injuries, and they are counted among the number who died. An elderly Lebanese man, a custodian/vendor who was known to work and sleep in his concession stand next to the building, was also killed in the first blast. The explosives used were later estimated to be equivalent to as much as of TNT.

Minutes later, a second suicide bomber struck the nine-story Drakkar building, a few kilometers away, where the French contingent was stationed. Fifty-five paratroopers from the 1st Parachute Chasseur Regiment and three paratroopers of the 9th Parachute Chasseur Regiment were killed and 15 injured. It was the single worst French military loss since the end of the Algerian War.Wright, Robin, Sacred Rage, Simon and Schuster, 2001, p. 72 The wife and four children of a Lebanese janitor at the French building were also killed, and more than twenty other Lebanese civilians were injured.

A group called Islamic Jihad claimed responsibility for the bombings and said that the aim was to force the MNF out of Lebanon. According to Caspar Weinberger, then United States secretary of defense, there is no knowledge of who did the bombing. Some analysis highlights the role of Hezbollah and Iran, calling it "an Iranian operation from top to bottom". There is no consensus on whether Hezbollah existed at the time of bombing. The attacks eventually led to the withdrawal of the international peacekeeping force from Lebanon, where they had been stationed following the Palestine Liberation Organization (PLO) withdrawal in the aftermath of Israel's 1982 invasion of Lebanon.

Beirut: June 1982 to October 1983 Timeline 6 June 1982 – Israel invades Southern Lebanon: Operation "Peace for Galilee."

23 August 1982 – Bachir Gemayel is elected to be Lebanon's president.

25 August 1982 – A MNF of approximately 400 French, 800 Italian soldiers and 800 US Marines of the 32nd Marine Amphibious Unit (MAU) are deployed in Beirut as part of a peacekeeping force to oversee the evacuation of Palestine Liberation Organization (PLO) guerrillas.

10 September 1982 – The PLO retreats from Beirut under MNF protection. Subsequently, the 32nd MAU is ordered out of Beirut by the President of the United States.

14 September 1982 – Lebanon's President, Bachir Gemayel, is assassinated.

16 September to 18 September 1982 – The Sabra and Shatila massacre occurs.

19 September 1982 – The destroyer USS John Rodgers and nuclear cruiser USS Virginia, operating off the coast of Beirut, conduct a naval bombardment onto the town of Suk al Gharb, in the hills overlooking Beirut, in support of the Lebanese Army, after it is nearly overrun by Syrian-backed Druze militiamen and Palestinian guerrillas. Over 300 rounds of 5" shells are fired to suppress the attack.New York Times, Page 1, September 20, 1982

20 September 1982 – The Beirut residence of the U.S. ambassador is shelled; for a second day U.S. naval ships again conduct counter fire operations.New York Times, Page 1, September 21, 1982

21 September 1982 – Bachir Gemayel's brother, Amine Gemayel, is elected to be Lebanon's president.

29 September 1982 – The 32nd MAU is redeployed to Beirut (primarily at the BIA) rejoining 2,200 French and Italian MNF troops already in place.

30 October 1982 – The 32nd MAU is relieved by the 24th MAU.

15 February 1983 – The 32nd MAU, redesignated as the 22nd MAU, returns to Lebanon to relieve the 24th MAU.

18 April 1983 – The U.S. Embassy bombing in Beirut kills 63, of whom 17 are Americans.

17 May 1983 – The May 17 Agreement is signed.

30 May 1983 – The 24th MAU relieves the 22nd MAU.

Mission

On June 6, 1982, the Israel Defense Forces (IDF) initiated Operation "Peace for Galilee" and invaded Lebanon in order to create a 40 km buffer zone between the PLO and Syrian forces in Lebanon and Israel. The Israeli invasion was tacitly approved by the U.S., and the U.S. provided overt military support to Israel in the form of arms and materiel. The U.S.' support for Israel's invasion of Lebanon taken in conjunction with U.S. support for Lebanese President Bachir Gemayel and the Lebanese Armed Forces (LAF) alienated many.

Bachir Gemayel was the legally elected president, but he was a partisan Maronite Christian and covert associate of Israel. These factors served to disaffect the Lebanese Muslim and Druze communities. This animosity was made worse by the Phalangist, a right-wing, largely Maronite-Lebanese militia force closely associated with President Gemayel. The Phalangist militia was responsible for multiple, bloody attacks against the Muslim and Druze communities in Lebanon and for the 1982 atrocities committed in the PLO refugee camps, Sabra and Shatila by Lebanese Forces (LF), while the IDF provided security and looked on.

The Phalangist militia's attacks on Sabra and Shatila were purportedly a response to the September 14, 1982, assassination of President-elect Bachir Gemayel. Amine Gemayel, Bachir's brother, succeeded Bachir as the elected president of Lebanon, and Amine continued to represent and advance Maronite interests.

All of this, according to British foreign correspondent Robert Fisk, served to generate ill will against the MNF among Lebanese Muslims and especially among the Shiites living in the slums of West Beirut. Lebanese Muslims believed the MNF, and the Americans in particular, were unfairly siding with the Maronite Christians in their attempt to dominate Lebanon. As a result, this led to artillery, mortar, and small arms fire being directed at MNF peacekeepers by Muslim factions. Operating under the peacetime rules of engagement, MNF peacekeepers – primarily U.S. and French forces – used minimum use of force as possible in order to avoid compromising their neutral status.DOD Commission on Beirut International Airport December 1983 Terrorist Act Until October 23, 1983, there were ten guidelines issued for each U.S. marine member of the MNF:

When on post, mobile or foot patrol, keep loaded magazine in weapon, bolt closed, weapon on safe, no round in the chamber. Do not chamber a round unless instructed to do so by a commissioned officer unless you must act in immediate self-defense where deadly force is authorized. Keep ammo for crew-served weapons readily available but not loaded in the weapon. Weapons will be on safe at all times. Call local forces to assist in self-defense effort. Notify headquarters. Use only minimum degree of force to accomplish any mission. Stop the use of force when it is no longer needed to accomplish the mission. If you receive effective hostile fire, direct your fire at the source. If possible, use friendly snipers. Respect civilian property; do not attack it unless absolutely necessary to protect friendly forces. Protect innocent civilians from harm. Respect and protect recognized medical agencies such as Red Cross, Red Crescent, etc.

The perimeter guards at the U.S. Marine headquarters on the morning of October 23, 1983, were in full compliance with rules 1–3 and were unable to shoot fast enough to disable or stop the bomber (see Bombings: Sunday, October 23, 1983 below).

In 1982, Iran established a base in the Syrian-controlled Beqaa Valley in Lebanon. From that base, Iran's Islamic Revolutionary Guard Corps (IRGC) trained and equipped the recently created group, Hezbollah, formed to resist and fight back against the Israeli invasion and occupation of southern Lebanon. Some analysts believe the newly formed Islamic Republic of Iran was heavily involved in the bomb attacks and that a major factor leading it to orchestrate the attacks on the barracks was America's support for Iraq in the Iran–Iraq War and its extending of $2.5 billion in trade credit to Iraq while halting the shipments of arms to Iran.Anthony H. Cordesman, The Iran-Iraq war and Western Security, 1984–1987: Strategic Implications and Policy Options, Janes Publishing Company, 1987

A few weeks before the bombing, Iran warned that providing armaments to Iran's enemies would provoke retaliatory punishment.For Iran's threat of retaliatory measures; see Ettela'at, 17 September 1983; Kayhan, 13 October 1983; and Kayhan, 26 October 1983, quoted in Ranstorp, Magnus, Hizb'allah in Lebanon : The Politics of the Western Hostage Crisis, New York, St. Martins Press, 1997, p. 117 On September 26, 1983, "the National Security Agency (NSA) intercepted an Iranian diplomatic communications message from the Iranian intelligence agency, the Ministry of Information and Security (MOIS)," to its ambassador, Ali Akbar Mohtashemi, in Damascus. The message directed the ambassador to "take spectacular action against the American Marines." The intercepted message, dated September 26, would not be passed to the Marines until October 26: three days after the bombing.

Much of what is now public knowledge of Iranian involvement in the bombings, such as the purported supply of PETN by Iran and the suicide bomber's name and nationality, was not revealed to the public until the 2003 trial, Peterson, et al v. Islamic Republic, et al. Testimony by Admiral James "Ace" Lyon's, U.S.N. (Ret), and FBI forensic explosive investigator Danny A. Defenbaugh, plus a deposition by a Hezbollah operative named Mahmoud (a pseudonym) were particularly revealing.

Incidents On July 14, 1983, a Lebanese Armed Forces patrol was ambushed by Lebanese Druze militia elements and from July 15–17, Lebanese troops engaged the Shia Amal militia in Beirut over a dispute involving the eviction of Shiite squatters from a schoolhouse. At the same time, fighting in the Shuf between the LAF and Druze militia escalated sharply. On July 22, Beirut International Airport (BIA), the headquarters of the U.S. 24th Marine Amphibious Unit (24th MAU), was shelled with Druze mortar and artillery fire, wounding three U.S. Marines and causing the temporary closure of the airport.

On July 23, Walid Jumblatt, leader of the predominantly Druze Progressive Socialist Party (PSP), announced the formation of a Syrian-backed "National Salvation Front" opposed to the May 17 Agreement. In anticipation of an IDF withdrawal from the Alayh and Shuf districts, fighting between the Druze and LF and between the Druze and LAF, intensified during the month of August. Druze artillery closed the BIA between 10 and 16 August, and the Druze made explicit their opposition to LAF deployment in the Shuf. The LAF also clashed with the Amal militia in Beirut's western and southern suburbs.

As the security situation deteriorated, US positions at BIA were subjected to increased fire. On August 10 and 11, an estimated thirty-five rounds of mortar and rocket fire landed on US positions, wounding one marine. On August 28, in response to constant mortar and rocket fire upon US positions, US peacekeepers returned fire for the first time. On the following day, US artillery silenced a Druze battery after two marines were killed in a mortar attack. On August 31, the LAF swept through the Shia neighborhood of West Beirut, establishing temporary control over the area.