The blast rippled across Beirut just after dawn, throwing Khodr Hammoud out of bed and stumbling to his front door.

Gazing across the packed houses of the Shiite-populated slums east of Beirut airport, the young Shiite resident saw a huge plume of smoke rising into the pale sky.

A suicide bomber had just hit the barracks housing the US Marines beside Beirut airport. The blast 25 years ago on Thursday killed 241 Americans, almost all of them marines, in what remains the highest fatality toll for the Corps in a single day since Iwo Jima in World War II.

"When I heard that marines had been blown up, I couldn't believe it," says Mr. Hammoud. "We didn't think of [the Marines] as an enemy then like we do now."

The bombing that left the Reagan administration's Lebanon ambitions in tatters continues to reverberate today in shaping US diplomatic, political, and counterinsurgency policies toward Lebanon and the Middle East.

"It was a turning point in asymmetrical warfare, especially in the Middle East," says Timur Goksel, a security analyst and former long-serving United Nations peacekeeper in south Lebanon. "All those people who couldn't fight powerful armies such as the United States suddenly found an easy way of balancing strength on the ground. That was the beginning and we have been seeing it ever since."

The attack exposed the vulnerabilities of even a superpower such as the US, which found itself unable to retaliate against its shadowy and anonymous adversaries, and ushered in a new era of grand-scale bombings – the World Trade Center in 1993, the Khobar Towers in Saudi Arabia in 1996, the US embassies in Kenya and Tanzania in 1998 – that reached its apogee on Sept. 11, 2001.

"In terms of significance it forced us out of Lebanon – and a cascade of attacks of the same nature forced us into the crusader castles that we live in today in the Middle East," says Robert Baer, a former Central Intelligence Agency field agent who operated in Beirut in the 1980s.

The tactic was first used against the Americans in Beirut six months earlier when the US Embassy was destroyed in a suicide car bomb blast that claimed the lives of 57 people. But the first suicide car-bomb attack had occurred three years earlier when a militant from the Dawa Party, an Iraqi opposition group, car-bombed the Iraqi Embassy in Beirut.

While the 1983 barracks bombing was not the first attack using an explosives-packed vehicle, it was the first to have a major political consequence.

The bombing broke the spine of US policy toward Lebanon, and four months later the marines had been withdrawn from the country, effectively ending Washington's involvement in war-torn Lebanon for another five years.

Iran is generally cited as the chief suspect in ordering the attack, with Lebanese Shiites carrying it out. Lebanon's militant Hezbollah is often blamed, although it was still coalescing in Lebanon's Bekaa Valley at the time and would not formally announce its existence for another 17 months.

Imad Mughniyah, who was posthumously identified as Hezbollah's top military commander following his assassination in Damascus, Syria, in February, often is linked to the attack on the marines. To this day no evidence has publicly emerged to clarify who was responsible.

"We never even narrowed down the name," says Mr. Baer.

Al Qaeda leader Osama bin Laden has cited Israel's 1982 invasion of Lebanon as a key motivator for his decision to participate in the struggle against the Soviet occupation of Afghanistan in the 1980s. And it was Mr. bin Laden who emulated and adapted the suicide car bomb tactic first used by Shiites in Lebanon.

The marines were based around Beirut airport at the southern end of the city, barricaded in a bombed-out building known as the BLT – the Battalion Landing Team. The Americans – part of a multinational peacekeeping mission – generated little public hostility initially – most Lebanese were happy at the sight of foreign soldiers helping maintain stability in a country that had been at war for eight years.

But the US found itself in the months ahead sucked into Lebanon's chaos as it lent ever-greater support to the Lebanese government and Army then battling pro-Syrian Druze militiamen. The US came to be regarded as just another faction in the civil war.

By Oct. 22, the 1,600-strong Marine force had suffered seven fatalities and 47 wounded. The next morning at 6.30 a.m., while the American servicemen were asleep in the BLT, a truck accelerated though the barbed-wire obstacles and crashed into the lobby of the building and exploded. At the same moment, another suicide bomber drove into the French paratroop headquarters, killing 58 people.

The Federal Bureau of Investigations concluded that the bomb used to flatten the Marine barracks – which had an explosive force comparable to 12,000 pounds of TNT – was the largest ever nonnuclear blast investigated by explosives experts.

In February 1984, the Lebanese Army collapsed, Reagan ordered the withdrawal of the marines and, 11 days later, the last of them had departed Lebanon, bringing to an end what Caspar Weinberger, the defense secretary, described as a "particularly miserable affair."