The 1st Marine Division is a Marine infantry division of the United States Marine Corps headquartered at Marine Corps Base Camp Pendleton, California. It is a subordinate unit of the I Marine Expeditionary Force (I MEF). It is the oldest and largest active duty division in the United States Marine Corps, representing a combat-ready force of more than 19,000 men and women. It is one of three active duty divisions in the Marine Corps today and is a multi-role, expeditionary ground combat force. It is nicknamed "The Old Breed".