2nd Battalion, 9th Marines (2/9) was an infantry battalion of the United States Marine Corps. Formed during World War I, the unit played an instrumental role in the defeat of the Japanese forces in the Battles of Guam and Iwo Jima during the World War II. The battalion distinguished itself in the defense of Khe Sanh during the Vietnam War, and later participated in an ill fated invasion of Koh Tang Island in Southeast Asia, with the intention of rescuing the crew of the SS Mayaguez. During Operation Desert Storm, the battalion served as the lead battalion for the III Marine Expeditionary Force (III MEF).