In 1982 a total of 241 military personnel, including 220 Marines, were killed by a truck bomb in Beirut, Lebanon. What was the reason for this attack against an American military base

To force the Multinational force out of Lebanon

Explanation: On October 23, 1983, two truck loaded with bombs struck buildings in Beirut, Lebanon, housing American and French service members of the Multinational Force in Lebanon. This was a joint military peacekeeping operation during the Lebanese Civil War. The attack killed 307 people including 241 American and 58 French military personnel, six civilians as well as two attackers.

A group called Islamic Jihad claimed responsibility for the bombings, claiming the aim was to force the Multinational force out of Lebanon. According to the United States Secretary of Defense however, there is no knowledge of who was actually responsible for the bombing, just some speculations. The attacks eventually led to the withdrawal of the international peacekeeping force from Lebanon.