The United States Marine Corps is organized within the Department of the Navy, which is led by the Secretary of the Navy (SECNAV). The most senior Marine commissioned officer is the Commandant of the Marine Corps, responsible for organizing, recruiting, training, and equipping the Marine Corps so that it is ready for operation under the command of the unified combatant commanders. The Marine Corps is organized into four principal subdivisions: Headquarters Marine Corps, the Operating Forces, the Supporting Establishment, and the Marine Forces Reserve.
The Operating Forces are further subdivided into three categories: Marine forces assigned to Unified Combatant Commands, Marine Corps Security Forces guarding naval installations, and Marine Security Guard detachments at American embassies. Under the "Forces for Unified Commands" memo, Marine forces are assigned to each of the regional unified combatant commands at the discretion of the Secretary of Defense and with the approval of the President. Since 1991, the Marine Corps has maintained component headquarters at each of the regional unified combatant commands.
Marine Corps Forces are further divided into Marine Forces Command (consisting of II Marine Expeditionary Force) and Marine Forces Pacific (I Marine Expeditionary Force and III Marine Expeditionary Force). The commander of the former also serves as commanding general for Fleet Marine Force, Atlantic, Marine Corps Forces, Europe, Marine Corps Forces, South, Marine Corps Forces, Strategic, and Marine Corps Installations East; while the latter serves as commander of Fleet Marine Force, Pacific, Marine Forces Central Command, and Marine Corps Installations West.
The Supporting Establishment includes Combat Development Command, Recruit Depots, Marine Corps Logistics Command, Marine Bases & Air Stations, Marine Corps Recruiting Command, and the United States Marine Band.