—flew hijacked planes into the World Trade Center and the Pentagon on Sept. 11, 2001, killing nearly 3,000 people.

In those cases and dozens more, terrorists exploited unconventional methods and Western openness. And in almost every case until Sept. 11, the U.S. military response was minimal.

For bin Laden and other terrorist leaders, Beirut showed how to attack a larger force and inflict the maximum damage — physical and psychological. Terrorism experts say manuals found in al-Qaida's Afghanistan training camps were filled with references to Beirut.

"The fact is, today, the people who ran that operation are heroes" among terrorist groups, "and nothing has ever been done against them," said Lehman, the former Navy secretary. "Not retaliating was a terrible blunder."

While terrorists took their lessons from Beirut, the Pentagon learned more about when to send troops and how to protect them.

"Culturally, it changed the military," said Phil Anderson, a former Marine and terrorism expert.

Weinberger summed up the lessons in a 1984 speech. The main points of what became known as the "Weinberger Doctrine" were restated after the first Gulf War by Colin Powell, then-chairman of the Joint Chiefs of Staff: The United States should commit troops only when vital national interests are at stake, only as a last resort, and with overwhelming force.

"It has to be for a sufficiently important cause," Weinberger told The Charlotte Observer.

The doctrine has been modified — and sometimes ignored — over the years, but the Beirut lessons still had a major impact:

Commanders insisted on more clearly defined missions with sufficient force to carry them out and a way to determine when troops could go home.

"You don't halfstep it," said Jay Farrar, a former Marine captain who served in Beirut and is now a military expert for the Center for Strategic and International Studies. "You deal swiftly and with a tremendous amount of force."

Policymakers realized that "presence" is not a mission, and commanders became increasingly reluctant to commit troops to peacekeeping efforts unless they were welcomed by all sides. Armitage cited the recent example of the U.S. role in Liberia, when both sides in a civil war requested American troops.

The concept of "force protection" came of age after the barracks attack. Rules of engagement are less limiting, and security around U.S. forces is tighter.

"We go in heavy," said P.J. Crowley, a retired Air Force colonel and former special assistant for national security affairs to President Clinton. "We have a plan for protecting our forces."