3rd Battalion 5th Marines (3/5) is an 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...

 battalion in 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...

. The battalion, nicknamed "Dark Horse", is based out of Marine Corps Base Camp Pendleton

Marine Corps Base Camp Pendleton

Marine Corps Base Camp Pendleton is the major West Coast base of the United States Marine Corps and serves as its prime amphibious training base...

, California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 and consists of approximately 1,000 Marines and Sailors. They fall under the command of the 5th Marine Regiment and the 1st Marine Division.
3rd Battalion, 5th Marines, along with the 5th Marine Regiment, was first organized on June 8, 1917 as the United States prepared for World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

.
The battalion was composed of four companies; the 16th, 20th, 45th and 47th.