1983 Beirut barracks bombings
On October 23, 1983, suicide bombers, by unknown actors often believed associated with Iran or Iranian-backed Hezbollah, attacked French and United States barracks, near Beirut International Airport. They were occupied by members of a United Nations Multinational Force, with participation from U.S. military forces were inserted into Lebanon on 29 September 1982 as part of a Multinational Force composed of U.S., French, Italian and, somewhat later, British troops. It killed the lives of 241 American and 58 French soldiers, with many casualties. Since it was conducted against a military objective, it may not meet the strict definition of terrorism, but the term is widely used due both to the means of attack, and that it was carried out by non-national actors. It was not principally intended to terrorize a civilian population.
Further complicating the designation is that there is no consensus on who sponsored the attacks. A US court did find Iran responsible, which would make it an attack by a nation-state on military personnel of two other nation-states. In 2001, former Secretary of Defense Caspar Weinberger stated: "But we still do not have the actual knowledge of who did the bombing of the Marine barracks at the Beirut Airport, and we certainly didn't then." 
Earlier, on April 18, the U.S. embassy had been blown up by a truck bomb. By August, the Multinational Force positions were under regular attack by car bombs and small arms fire. In September, U.S. warships gave gunfire support to Lebanese Armed Forces in the Beirut International Airport area.
Attack on the U.S barracks
A large Mercedes truck accelerated through minimal traffic barriers around the U.S. barracks, while the sentry struggled to load his M-16 rifle, unloaded under the existing rules of engagement. Crashing through barriers and another guard post, with the sentry able to fire a few shots at the driver, it broke into the building center and exploded.  At best, rifle fire could have killed the driver; the explosives might have detonated outside if, as is common practice, they were controlled by a "dead man switch". The weapons and barriers available were unable to stop a large truck.
U.S. troops originally arrived to a generally friendly response on 29 September 1982 as part of a Multinational Force composed of U.S., French, Italian troops; British soldiers later joined. It was intended to establish security in the Beirut area.
Rules of engagement for the Marines were restrictive; they could not set up what would be considered today a safe perimeter against truck bombs. They carried rifles that had to be loaded before use; there were no heavier weapons that could deflect a truck or destroy its engine. 
The Commission termed this a terrorist attack, and raised questions about the intelligence support available to it. It
Intelligence provided a good picture of the broad threat facing the USMNF [US multinational force] in Lebanon. Every intelligence agency in the national community and throughout the chain of command disseminated a great amount of analysis and raw data. Key Defense officials and the military chain of command ere alert to, and concerned with, the insights it provided them. There was an awareness of the existing dangerous situation at every level, but no one had specific information on how, where and when the threat would be carried out. Throughout the period of the USMNF presence in Lebanon, intelligence sources were unable to provide proven, accurate, definitive information on terrorist tactics against our forces. This shortcoming held to be the case on 23 October 1983. The terrorist threat was just one among many threats facing the USMNF from the many factions armed with artillery, crew served weapons and small arms....
The USMNF was operating in an urban environment surrounded by hostile forces without any way of pursuing the accuracy of data in order to head off attack. The intelligence structure should be reviewed from both a design and capabilities standpoint. We need to establish ourselves early in a potential trouble spot and find new techniques to isolate and penetrate our potential enemies. Once established, our military forces (and especially ground forces) need to have aggressive, specific intelligence to give the commander the hard information he needs to counter the threats against his force. U.S. intelligence is primarily geared for the support of air and naval forces engaged in nuclear and conventional warfare. Significant attention must be given by the entire U.S. intelligence structure to purging and refining of masses of generalized information into intelligence analysis useful to small unit ground commanders.
- Weinberger, Caspar. Interview: Caspar Weinberger, PBS Frontline, 2001.
- Randy Gaddo, "25 Years Ago: The 1983 Beirut Bombing: Who Did It and How It Has Affected History", Leatherneck
- Long, Robert L.J.; Robert J. Murray & Lawrence F. Snowden et al. (20 December 1983), Report of the DoD Commission on Beirut International Airport Terrorist Act, October 23, 1983
- Long, Robert L.J.; Robert J. Murray & Lawrence F. Snowden et al. (20 December 1983), Part Four. Intelligence, Report of the DoD Commission on Beirut International Airport Terrorist Act, October 23, 1983, at 57-66