A truck bomb ripped through the U.S. Marine barracks in Beirut 20 years ago this week, marking the first major assault in a two-decade terrorist war of embassy bombings and plane hijackings that culminated on Sept. 11, 2001.

The shocking attack killed 241 U.S. servicemen in a single strike — more than died on the deadliest day of fighting in Vietnam, this year's invasion of Iraq or the entire 1991 Persian Gulf War.

And it gave terrorists a major victory. The bombing drove the military from its peacekeeping mission in Lebanon and provided a blueprint for attacking Americans. The retreat of U.S. forces inspired Osama bin Laden and sent an unintended message to the Arab world that enough body bags would prompt Western withdrawal, not retaliation.

"There's no question it was a major cause of 9/11," said John Lehman, the then-secretary of the Navy, who today is a member of the independent commission investigating the Sept. 11 attacks.

"We told the world that terrorism succeeds."

About 2,000 Beirut veterans and family members will gather Thursday at Camp Lejeune in eastern North Carolina, where most were stationed in 1983. They will mourn fallen comrades and remember a doomed mission.

At best, they believe, the world has forgotten their sacrifices. At worst, they fear they'll always be considered a failure — and the painful lessons of their tragedy will be ignored.

"It was such a useless, fruitless thing," says Brian Kirkpatrick, a Beirut survivor who crawled his way out of the rubble. "We gained nothing. We lost everybody."

But in the halls of the Pentagon and the State Department, Beirut has not been forgotten, Deputy Secretary of State Richard Armitage told The Charlotte Observer. Many of the leaders from 20 years ago — who now serve President Bush — work to avoid a repeat of the disaster as they plan the military missions of today.

Bush reminded Americans of the tragedy in a prime-time speech last month. He urged the country to prepare for a long and costly effort to rebuild Iraq and not to repeat the mistake of leaving before the job was done.

"What would happen if we left this business unfinished," Armitage said, "is an Iraq that would become more of a threat — sort of an Iraq unchained."

The Beirut bombing taught the United States more about protecting troops and picking battles. Using the military for peacekeeping, leaders learned, can be just as hazardous as fighting a well-defined enemy.

But as the U.S. death toll in Iraq rises, critics of the Bush administration question whether those lessons are being heeded, or if the United States has been set up for another failure at the cost of American lives.

In 1983, Defense Secretary Caspar Weinberger didn't want the Marines in Beirut.

They'd gone in the year before to calm fears that Lebanon's civil war could spark a battle engulfing the entire Middle East. The Marines' role was to evacuate Palestinian fighters and prevent an invasion from neighboring Israel.

U.S. diplomats promised the safety of Palestinian families who remained. But after the Marines finished their job and withdrew in 1982, thousands of Palestinians — largely women, children and the elderly — were massacred by Israeli-backed Lebanese militia.

An embarrassed State Department persuaded President Ronald Reagan to send the Marines back, hoping their mere presence in Lebanon would prevent further bloodshed and salvage the peace plan. Weinberger fought the decision.