3rd Battalion, 23rd Marines was activated on July 22, 1942, at what is now Marine Corps Air Station New River, North Carolina. In July 1943, the battalion relocated to Marine Corps Base Camp Pendleton, California. In August 1943, they were assigned to 4th Marine Division. This division was formed by the organization and redesignation of several other units. The 23rd Marine Regiment began as infantry detached from the 3rd Marine Division, and an artillery battalion of the 12th Marines became the genesis of the 14th Marines and engineer elements of the 19th Marines formed the nucleus of the 20th Marines. In March the 24th Marine Regiment was organized, and then in May it was split in two to supply the men for the 25th Marines.