Unified Ground Command

The Unified Ground Command (abbreviated UNICOM or UniCom) is a United Nations Space Command organization tasked with coordinating the terrestrial operations of the Army, the Marine Corps, and the Air Force. UNICOM's roles include, but are not limited to, disaster relief, infantry operations, search and reconnaissance, and other "ground pounder" duties. Special operations within the organization's oversight are coordinated by UNICOM's dedicated Unified Special Warfare Command. The Marine Corps' Orbital Drop Shock Troopers are an exception as they are instead controlled by Naval Command's Naval Special Weapons.

Structure
Ground forces under UNICOM's authority are loosely modeled on the organization of the 21st century United States military. The force numbers given in the descriptions below are ideals, rarely matched exactly in the field. At the higher levels, these numbers include a significant amount of support personnel such as logistics and administration.

Fireteam
A fireteam is the smallest unit of UNICOM's force organization. It contains a total of three soldiers plus a fireteam leader, usually a corporal or sometimes a lance corporal in the Marine Corps.

Squad
A squad contains three fireteams led by a sergeant for a total of thirteen members.

Platoon
A platoon normally contains three squads (39 personnel); plus the platoon leader, a Second or First Lieutenant; the Platoon Sergeant (the second-in-command), usually a Sergeant or Staff Sergeant; and a medic, for a total of forty-two men.

Company
A company contains four platoons plus a group of personnel assigned as "Company Headquarters Section" (164 personnel). It is commanded by a captain with a company first sergeant. The twelve companies of a brigade or regiment are named phonetically "A" (Alpha) through "M" (Mike) with the exclusion of "J" (Juliet). First Battalion consists of companies A through D, Second Battalion is companies E through H, and so forth.

A typical infantry company would have three infantry platoons and one heavy weapons platoon, but these numbers are flexible depending on the unit type and mission. "T" companies, for example, eschew this model and mix their heavy weapons personnel into each platoon so all the platoons have one heavy weapon squad.

Battalion
A battalion contains four companies plus a group of command and staff personnel assigned as Headquarters Company (eight hundred personnel). It is commanded by a lieutenant colonel with a battalion sergeant major; this would be a sergeant major in the Marine Corps or a command sergeant major in the Army.

Regiment (Marine Corps) or brigade (Army)
A Marine Corps regiment contains three battalions, plus a group of personnel assigned as Headquarters Company (3,000 personnel) commanded by a colonel with a sergeant major as senior non-commissioned officer.

The Army uses permanent brigades in lieu of regiments, though their unit compositions are largely similar to Marine regiments. Army brigades likewise contain an average of three battalions and their attendant personnel.

Division
A division contains four regiments plus a group of personnel assigned as Headquarters Battalion (14,000 personnel) commanded by a major general. The extra personnel in the force number given are support units, including medical, combat engineering, maintenance, and military police.

Production note
The Halo Encyclopedia claims that UNICOM's unit composition and rank hierarchy are based on those of the United States Marine Corps. This has since been negated as the Army uses the rank of specialist (E-4) and fields permanent brigades. These are both United States Army traditions that are not shared by the Marine Corps.