Marine Command (MARCOM) - Responsible for the Marine Corps, the Marine Command handles all of the branch's operations, which mostly involve offensive UNSC infantry operations and defense/security of Navy structures and bases, working closely alongside and generally being subordinate to FLEETCOM.
Army Command (ARCOM) - The Army Command is in charge of managing the Army and its operations, mainly garrisoning UNSC worlds or, on rare occasions, invasion operations alongside the Marine Corps, though the latter is very uncommon.
Colonial Militia (CM) -
Air Force Command (AFCOM) - Though in decline due to many of its personnel and aircraft going to other branches of the UNSC Defense Force, the Air Force still exists and is managed by the Air Force Command, managing dropships, gunships and fighter jets. Thus, they are responsible for carrying out air strikes, and transport marine and army troops and equipment.