At 6:22 on Sunday morning Oct. 23, 1983, a 19-ton yellow Mercedes stake-bed truck entered a public parking lot at the heart of Beirut International Airport. The lot was adjacent to the headquarters of the U.S. 8th Marine Regiment’s 1st Battalion, where some 350 American service members lay asleep in a four-story concrete aviation administration building that had been successively occupied by various combatants in the ongoing Lebanese Civil War. Battalion Landing Team 1/8 was the ground element of the 1,800-man 24th Marine Amphibious Unit (MAU), which had deployed to Lebanon a year earlier as part of a multinational peacekeeping force also comprising French, Italian and British troops. Its mission was to facilitate the withdrawal of foreign fighters from Lebanon and help restore the sovereignty of its government at a time when sectarian violence had riven the Mediterranean nation.

The force represented a neutral and stabilizing presence, and the Lebanese people welcomed its arrival. But it soon got caught up in the burgeoning conflict, and what peacekeepers had described as a benign environment became decidedly hostile. Indeed, on April 18, 1983, a suicide bomber had detonated an explosives-laden delivery van outside the U.S. Embassy in Beirut, killing 63 people, including 17 Americans. In subsequent weeks Druze and Shiite militias sporadically fired artillery shells, mortar rounds and rockets at the Marines on or near their Beirut airport base. Still, the airport largely remained open, and, astoundingly, U.S. commanders seemingly did little to bolster base security.

It was an oversight that would have fatal consequences.

Marine sentries initially paid little attention to the Mercedes truck. Heavy vehicles were a common sight at the airport, and in fact the BLT was expecting one that day with a water delivery. The truck circled the parking lot, then picked up speed as it traveled parallel to a line of concertina wire protecting the south end of the Marine compound. Suddenly, the vehicle veered left, plowed through the 5-foot-high wire barrier and rumbled between two guard posts.

By then it was obvious the driver of the truck — a bearded man with black hair — had hostile intentions, but there was no way to stop him. The Marines were operating under peacetime rules of engagement, and their weapons were not loaded. Lance Corporal Eddie DiFranco, manning the sentry post on the driver’s side of the truck, soon guessed the driver’s horrifying purpose. “He looked right at me…smiled, that’s it,” DiFranco later recalled. “Soon as I saw [the truck] over here, I knew what was going to happen.” By the time he managed to slap a magazine into his M16 and chamber a round, the truck had roared through an open vehicle gate, rumbled past a long steel pipe barrier, threaded between two other pipes and was closing on the BLT barracks.

Sergeant of the guard Stephen Russell was alone at his sandbag-and-plywood post at the front of the building but facing inside. Hearing a revving engine, he turned to see the Mercedes truck barreling straight toward him. He instinctively bolted through the lobby toward the building’s rear entrance, repeatedly yelling, “Hit the deck! Hit the deck!” It was futile gesture, given that nearly everyone was still asleep. As Russell dashed out the rear entrance, he looked over his shoulder and saw the truck slam through his post, smash through the entrance and come to a halt in the midst of the lobby. After an ominous pause of a second or two, the truck erupted in a massive explosion — so powerful that it lifted the building in the air, shearing off its steel-reinforced concrete support columns (each 15 feet in circumference) and collapsing the structure. Crushed to death within the resulting mountain of rubble were 241 U.S. military personnel — 220 Marines, 18 Navy sailors and three Army soldiers. More than 100 others were injured. It was worst single-day death toll for the Marines since the World War II Battle of Iwo Jima.

At the sound of the blast Colonel Tim Geraghty, commander of the 24th MAU, ran outside his combat operations center. “[I found] myself engulfed in a dense gray fog of ash,” he recalled, “with debris still raining down.” His logistics officer, Major Bob Melton, then motioned to Geraghty and gasped, “My God, the BLT building is gone!” Rabbi Arnold Resnicoff, assistant chaplain for the U.S. Sixth Fleet, was in a nearby building when the explosion occurred and later recounted the horrific scene: “Bodies and pieces of bodies were everywhere. Screams of those injured or trapped were barely audible at first, as our minds struggled to grapple with the reality before us.” Sergeant Russell — who had watched the truck explode — had been blown through the air, knocked unconscious and injured but had managed to survive a gas-enhanced explosion experts later estimated had the destructive yield of 6 tons of TNT. The FBI Laboratory later described it as “the largest conventional blast” yet documented.

Within 10 minutes of the attack and a few miles north a suicide bomber in an explosives-packed pickup truck targeted a nine-story building housing soldiers from the 3rd Company of France’s 1st Parachute Chasseur Regiment. Guards shot and killed the driver, stopping the truck 15 yards shy of the building, but the terrorist still managed to trigger his device. Though only half as powerful as the bomb that had leveled the Marine compound, the second blast brought down the French barracks, killing 58 paratroopers—many of whom had been standing on outside balconies, trying to discern what had occurred at the U.S. base just down the coast.

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An unknown group calling itself Islamic Jihad claimed responsibility for the bombings. Investigators later concluded that Hezbollah — the Iranian- and Syrian-sponsored proxy army — had organized the attacks, which were significant in two ways, beyond the appalling death tolls. For one, they signaled an upswing in terrorism that has grown steadily worse over the last three decades. The attacks also made it clear extremists had altered their tactics. For years Islamic militants struck out at the West mostly with kidnappings — the 1979 abduction of more than 60 U.S. Embassy personnel in Tehran being the most conspicuous example. With the Beirut bombings such terrorists had raised the stakes, exhibiting a willingness to kill themselves in attacks aimed at slaughtering as many Westerners as possible.

Colonel Geraghty, who faced criticism for inadequate security at the Marine compound, suggested later the Beirut bombings marked the true start of the global war on terror. He drew a line from Lebanon through the al-Qaida attacks of Sept. 11, 2001, to the ongoing wars in Iraq and Afghanistan. “Who would have thought,” he said, “years later here we are [fighting] essentially the same crowd?”

According to an independent investigation of the Marine barracks bombing commissioned by the Department of Defense and chaired by retired Navy Admiral Robert L.J. Long, “Terrorist warfare, sponsored by sovereign states or organized political entities to achieve political objectives, is a threat to the United States that is increasing at an alarming rate. The catastrophe…demonstrates that the United States, and specifically the Department of Defense, is inadequately prepared to deal with this threat.”

When President Ronald Reagan ordered U.S. troops to Lebanon in the fall of 1982, the country previously dubbed an “Arab oasis” was on the boil — in the midst of a vicious civil war that persisted through the end of the decade.

The tiny Mediterranean country, wedged between Syria and Israel, had been fraught with sectarian tension since its creation under a League of Nations mandate after World War I. The Lebanon of antiquity comprised essentially Mount Lebanon, the 110-mile-long north-south mountain range that for more than a millennium had been home to the isolated Maronite Christians. Under the French-controlled mandate what had been a single administrative district of the Ottoman empire became two separate nations, Syria and Lebanon. Thrown together in the new Lebanon with the Maronites were Sunni and Shia Muslims, as well as the Druze — a Shia offshoot. It was a volatile mix to say the least. The Muslims did not want to be ruled by the Maronites and entertained hopes of being part of a greater Syria, while the Maronites staunchly opposed that concept.

The unwritten National Pact of 1943 established an unusual power-sharing arrangement. Under its terms the Lebanese president would be a Maronite Christian; the prime minister, a Sunni Muslim; the speaker of the parliament, a Shia Muslim; the deputy prime minister and deputy speaker of the parliament, Greek Orthodox Christians; the chief of the general staff, a Maronite; the chief of the army staff, a Druze. There would be six Christian members of parliament for every five Muslim members. The country would have an Arab, not Western, orientation but would not seek to unite with Syria. Despite a subsequent increase in Muslim numbers, the pact left Christians in disproportionate control of the government, army and parliament, which fostered discontent.

Though the pact stipulated Lebanon was to be a secular nation, sectarian rivalries became increasingly acute and led to power struggles. The country soon devolved into a patchwork of sectarian fiefdoms, none of which were much interested in cooperation with other groups or the weak central government. As the Long commission report put it, “There is no sense of national identity that unites all Lebanese or even a majority of the citizenry. What it means to be Lebanese is often interpreted in radically different ways by, for instance, a Sunni Muslim living in Tripoli, a Maronite Christian from Brummana, a Greek Orthodox Christian from Beirut.…” The report added: “The National Pact set forth what Lebanon was not. It was not an extension of Europe, and it was not part of a pan-Arab state. It did not establish in positive terms what Lebanon was.” As a prominent Lebanese journalist once put it, “Two negations do not make a nation.”

The 1948 establishment of the state of Israel further destabilized the region. Between then and the 1967 Six-Day War more than 100,000 Palestinians fled to southern Lebanon. In 1970, when Jordan’s military forcibly expelled Yasser Arafat and the Palestine Liberation Organization, many of its fighters also fled to southern Lebanon. From there PLO guerrillas staged raids into northern Israel, sparking bloody reprisal attacks. The Israeli-Palestinian conflict in turn stoked the ethnic embers in Lebanon. Lebanese Muslims (along with Syria) supported the Palestinians, while Lebanese Christians (aligned with Israel) opposed them. Fighting between factional militias ensued, and in 1976 Syria’s Ba’th regime sent troops into Lebanon to fight leftist militias. According to the Long commission report, “Lebanon lay crippled under the weight of de facto partition and partial occupation by Syria” — an occupation that would last almost 30 years.

Then came another major upheaval. On June 6, 1982, Israel invaded southern Lebanon to oust PLO militants from their enclave — an event the Long commission report described as a “fatal overload” for a country coming apart at the seams. Within days the Israel Defense Forces (IDF) were on the outskirts of Beirut. A U.S.-led diplomatic effort ultimately brokered a cease-fire between the PLO and Israel and an agreement that Palestinian and Syrian forces would evacuate the capital. In August, under the watchful eye of the multinational force, they did — and Western troops soon returned to their ships in the Mediterranean.

But Lebanon still seethed. On September 14 an unknown assassin bombed the headquarters of Phalange, the Christian Democratic party, killing more than two-dozen officials, including President-elect Bashir Gemayel, a Maronite. Two days later right-wing Phalangist militiamen entered the Sabra and Shatila refugee camps, ostensibly to root out PLO cells, then massacred hundreds of Palestinians while the IDF stood by. By month’s end the multinational force had reentered the country, presiding over an uneasy calm through early 1983. Then came the U.S. Embassy bombing. A month later Israel and Lebanon signed an agreement under which Israeli soldiers would withdraw from Lebanon, contingent on the withdrawal of the Syrian troops. While the Syrians did not leave, Israel unilaterally withdrew its troops. That move only sparked more fighting among competing militias.

While the American element of the multinational force professed to be neutral, it wasn’t — and perhaps couldn’t be. Within two months of their arrival in late 1982 U.S. troops were training Lebanese Armed Forces (LAF) personnel, and by the fall of 1983, as fighting between the LAF and Druze and Shiite militias became fierce, the Americans ratcheted up their support of government forces, partly out of concern for their own security. By then the threat level had markedly increased for Western troops. Commanders specifically identified Druze artillery positions in the hills near Suq-al-Gharb, a few miles east of and overlooking the airport, as a threat to the multinational force. On September 7 U.S. Navy F-14 Tomcats flew tactical reconnaissance missions, and the next day offshore destroyers shelled the Druze positions. Eleven days later U.S. destroyers provided direct gunfire support of the LAF at Suq-al-Gharb. The decision to attack Shiite and Syrian positions apparently was significant. By then, investigators concluded, “The image of the [multinational force], in the eyes of the factional militias, had become pro-Israel, pro-Phalange and anti-Muslim.…A significant portion of the Lebanese populace no longer considered [it] a neutral force.”

But did terrorists attack the American and French compounds in reaction to the naval bombardment at Suq-al-Gharb? Investigators could not find a direct link, but the prevalent view within the U.S. European Command was “there was some linkage between the two events.” In his memoir Geraghty wrote the terrorists had targeted the Beirut barracks out of their “obsessive hatred of the West and what we represent.” But he later asserted that “American support [for the Lebanese government] removed any lingering doubts of our neutrality, and I stated to my staff at that time that were we were going to pay in blood for this decision.” In interviews before the Long commission other civilian and military officials argued that specific factions wanted to force Western troops out of Lebanon, and “the bombing of the BLT headquarters building was the tactic of choice to produce that end.”

After two years of investigation U.S. intelligence organizations pinned the planning of the Beirut bombings on a Lebanese Shia named Imad Mughniyah — who became a notorious Hezbollah terrorist —while his cousin and brother-in-law, Mustafa Badreddine, actually built the bombs. “Badreddine developed a trademark technique,” explained Washington Post reporter Robin Wright, “of using gas [compressed butane] to enhance the power of already sophisticated explosives.” Investigators say the pair had organized the earlier attack on the U.S. Embassy and in 1984 began abducting Westerners, mostly Americans, off the streets of Beirut, killing some, while holding others hostage for years. Mughniyah, who was also implicated in the 1996 Khobar Towers bombing in Saudi Arabia, was said by experts to be the prototype of the modern terrorist. “Long before Osama bin Laden, there was Imad Mughniyah,” Bilal Saab, a Hezbollah expert at the Brookings Institution, told Wright. “He introduced catastrophic suicide terrorism and many other tactics now used widely by many groups throughout the region.”

Lebanese author Hala Jaber, in her 1997 book about Hezbollah, claims Iran’s ambassador to Syria, Ali Akbar Mohtashemi — a founder of Hezbollah — helped organize the Beirut bombings in consultation with Syrian intelligence. She asserts the Marine barracks bomb was prepared in eastern Lebanon’s Bekaa Valley, then under Syrian control. Hezbollah and the Syrian and Iranian governments have denied any role in the bombings, though in 2004 Iran reportedly erected a monument in Tehran to the attacks and its “martyrs.”

Two years after the Marine barracks bombing, a U.S. grand jury indicted Mughniyah for his role in the attack and other terrorist activities. He was high on the FBI’s list of most wanted terrorists. Still, he evaded capture for 25 years before meeting a fitting fate in 2008 — killed by a car bomb in Damascus. Reports claimed Israel’s Mossad intelligence service was responsible, perhaps with help from the CIA.

Meanwhile, leaning on a 1996 exception to the 1976 Foreign Sovereign Immunities Act that allows civil lawsuits in U.S. courts against states that sponsor terrorism, courts in the District of Columbia have awarded victims of the bombings and their families more than $10 billion in compensatory judgments against Iran. “It’s an alternative way of dealing with the scourge of state-sponsored terrorism,” plaintiffs’ attorney Joseph P. Drennan of Alexandria, Va., told Newsweek in 2014. But collecting any actual money for the victims has proved difficult, despite efforts to seize Iranian bank accounts in various countries. In 2015, however, Congress established a $1 billion fund for victims of state-sponsored terrorism, available as soon as they receive a federal court decision. The money was drawn from $8.9 billion in penalties paid by French multinational bank BNP Paribas for violating Western sanctions against Iran, Sudan and Cuba.

The Long commission reached many conclusions about the Marine barracks bombing, none of them sanguine. “The facts of political life in Lebanon,” its report noted, “[made] any attempt on the part of an outsider to appear nonpartisan virtually impossible.” It cited confusion over the actual purpose of the peacekeeping mission and who was to be responsible for the security of the Beirut airport. The commission’s report faulted MAU and BLT commanders for security measures “neither commensurate with the increasing level of threat confronting the [multinational force] nor sufficient to preclude catastrophic losses such as those suffered on the morning of 23 October 1983.” It faulted the BLT commander for billeting some 350 men—roughly one-quarter of the force—in a single structure, which “contributed to the catastrophic loss of life.” It also faulted him for “[modifying] prescribed alert procedures, degrading security of the compound.” The report faulted the MAU commander, and in effect everyone at U.S. European Command headquarters, for condoning procedures that “emphasized safety over security in directing that sentries…would not load their weapons.” Following the April embassy bombing the European Command modified rules of engagement at that compound, authorizing “prompt, forceful action against any unauthorized attempt to gain entry,” but the MAU commander had assumed peacetime rules of engagement remained in effect at the Marine compound. The report concluded the multinational force “was not trained, organized, staffed or supported to deal effectively with the terrorist threat in Lebanon…[and] much needs to be done to prepare U.S. military forces to deal with terrorism.”

In early 1984 President Reagan seems to have come to the same conclusion. By then the security situation in Lebanon had further deteriorated. The leader of the Amal Movement—the political party representing Lebanon’s Shia Muslims—had asked the Americans, French, British and Italians to leave, while Islamic Jihad had made new threats. On Feb. 7, 1984, little more than three months after the Beirut bombings, Reagan ordered the Marines to begin withdrawing from Lebanon. The next day, as if venting U.S. frustration, the battleship New Jersey fired almost 300 16-inch shells at Druze and Syrian artillery and missile positions—a nine-hour bombardment that, according to the Navy, was the “heaviest shore bombardment since the Korean War.” By month’s end most of the multinational force had withdrawn from Beirut, and in late July the last remaining troops of the 24th MAU left Lebanon. While the United States maintained scores of bases in the Middle East, some years passed before combat troops again put boots on the ground in the region.

Richard Ernsberger Jr. is a former senior editor of American History and Newsweek magazines. For further reading he recommends The Root: The Marines in Beirut, by Eric Hammel, and Peacekeepers at War, by Timothy J. Geraghty.

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