The US 1st Marine Division is a division of the US Marine Corps headquartered at the US Military Base at Camp Pendleton, California. This division is a ground combat element of the I Marine Expeditionary Force. The 1st Marines is the oldest and largest active duty division in the Marine Corps, representing a combat-ready force of 19,000 personnel. It's also one of three active duty divisions in the US Marine Corps today and is a multi-role, expeditionary ground combat force. It is nicknamed "The Old Breed".