3rd Battalion, 12th Marines (3/12) is an artillery battalion comprising four firing batteries and a Headquarters Battery. The firing batteries are not permanently assigned to the battalion- they are all on 6-month rotations in Okinawa from the 10th Marine Regiment and the 11th Marine Regiment (known as the Unit Deployment Program (UDP)). The battalion is stationed at Camp Hansen, Okinawa, Japan, and its primary weapon systems are the M777 lightweight howitzer with a maximum effective range of 30km, along with the M142 High Mobility Artillery Rocket System (HIMARS). The battalion utilizes the Unit Deployment Program to fill its four firing batteries with detachments from artillery units across the Marine Corps.
Provide direct support of the 3rd Marine Division in time of conflict. That support may come in the traditional fashion of artillery support to maneuver forces, or by providing batteries to serve as provisional rifle companies. They also have the secondary mission of being the primary providers of civil-military operations (CMO). CMO is defined as the activities of the commander that establish, maintain, influence, or exploit relations between military organizations, Government and civilian organizations and the civilian populace.