The 3rd Battalion, 9th Marines (3/9) is an infantry battalion

Battalion

A battalion is a military unit of around 300–1,200 soldiers usually consisting of between two and seven companies and typically commanded by either a Lieutenant Colonel or a Colonel...

 of the United States Marine Corps

United States Marine Corps

The United States Marine Corps is a branch of the United States Armed Forces responsible for providing power projection from the sea, using the mobility of the United States Navy to deliver combined-arms task forces rapidly. It is one of seven uniformed services of the United States...

. Formed during World War I it served until the early 1990s when it was redesignated as 3rd Battalion, 4th Marines

3rd Battalion 4th Marines

3rd Battalion, 4th Marines is an infantry battalion of the United States Marine Corps. Nicknamed "Thundering Third" it is known within the battalion as "Darkside" and the radio callsigns of the command and staff reflect this moniker. They are based at the Marine Corps Air Ground Combat Center...

(3/4) during a realignment and renumbering of the Marine Corps' infantry battalions, following the deactivation of the 9th Marine Regiment. The 3rd Battalion, 9th Marines was initially a subordinate unit of the 9th Marine Regiment, 3rd Marine Division, but was later operationally transferred to the 1st Marine Division as a subordinate unit of the 7th Marine Regiment where it remained until its redesignation as 3/4.

3/9 was reactivated under the 2nd Marine Division in May 2008 as the Marines expands as a result of the continuing War on Terror

War on Terror

The War on Terror is a term commonly applied to an international military campaign led by the United States and the United Kingdom with the support of other North Atlantic Treaty Organisation as well as non-NATO countries...

.
The battalion comprised three infantry

Infantry

Infantrymen are soldiers who are specifically trained for the role of fighting on foot to engage the enemy face to face and have historically borne the brunt of the casualties of combat in wars. As the oldest branch of combat arms, they are the backbone of armies...

 companies, one weapons company and a headquarters and service company:

• Headquarters & Service Company
• India Company
• Kilo Company
• Lima Company
• Weapons Company

3rd Battalion, 9th Marine Regiment was reactivated on 12 February 1942, at Camp Elliot, San Diego, California as part of the 2nd Marine Division.