3rd Battalion 1st Marines (3/1) 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...

 based out of Camp Horno on 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...

. Nicknamed the "Thundering Third", the battalion consists of approximately 1220 Marines and Sailors and falls under the command of the 1st Marine Regiment and the 1st Marine Division.

• Headquarters and Service Company
  Headquarters and Service Company
  A Headquarters and Service Company is a company sized military unit, found at the battalion level and higher in the United States Marine Corps. In identifying a specific headquarters unit, it is usually referred to by its abbreviation as H&S...
  
  
• Company I
• Company K
  Company K
  Company K is a 1933 novel by William March, first serialised in parts in the New York magazine Forum from 1930 to 1932, and published in its entirety by Smith and Haas on 19 January 1933, in New York. The book's title was taken from the Marine company that March served in during World War I...
  
  
• Company L
• Weapons Company
  Weapons company
  A weapons company, sometimes called a manoeuvre support company is a company-sized military unit attached to an infantry battalion to support the rifle companies of the battalion...
  
  

3rd Battalion, 1st Marines was activated on 1 March 1941 at Guantanamo Bay, Cuba

Cuba

The Republic of Cuba is an island nation in the Caribbean. The nation of Cuba consists of the main island of Cuba, the Isla de la Juventud, and several archipelagos. Havana is the largest city in Cuba and the country's capital. Santiago de Cuba is the second largest city...

 and assigned to the 1st Marine Division.