UNSC Army Division Structure

The division is the UNSC Army's primary tactical unit, serving as the main building block of Army force structure. They are large all-arms units capable of conducting a wide range of tasks with their organic assets, though they still require augmentation from assets held at echelons above division in order to succeed in some difficult tasks. Separate brigades supplement divisions as the Army's secondary tactical unit.
== Structure ==
Tables of Organization & Equipment (TOEs) promulgated by Army headquarters dictate the doctrinal structure of every Army unit. However, reality is almost never a perfect match to doctrine. Due to unusual operational needs, manpower shortages, and differences in available equipment, most divisions deviate from the prescribed TOEs in small, or sometimes large, ways. Army headquarters accepts this and sometimes writes official doctrine for such deviations. Most divisions follow the main prescribed TOEs closely, but are invariably below authorized strengths. 
Army divisions have been restructured many times throughout the UNSC's history. Most recently, the Human-Covenant War triggered a shift from Insurrection-era divisions optimized for occupation duties with low-intensity combat to wartime designs that sought to fight large-scale battles under enemy air and space superiority. During the 26th century, most changes to Army divisions have remained relatively small, with few sweeping transformations. The information below reflects division structures as of 2558.
=== General Template ===
Army divisions are categorized into a few basic types, with some subtypes, defined primarily by the platforms they use to move and fight. This ensures that every division is able to move as one unit and limits the complexity of their supply chains. Every major type of division is commanded by a Major General and follows the same overall template.
The maneuver component consists of ten line battalions, which are task-organized into three brigades. TOEs prescribe a default arrangement for which battalions serve under which brigades, but allow division & higher commanders a free hand to rearrange this. Each brigade headquarters is commanded by a Colonel and comes with a headquarters battalion (HHBN) containing anti-tank, recon & security, and signal companies. One cavalry squadron provides recon & security for the division.
The divisional artillery is commanded by a Brigadier General and consists of five firing battalions. Three battalions are typically attached to the maneuver brigades, while the other two use longer-ranged weapons and usually remain under the division commander's control. 
The aviation component varies in size depending on division type, and is commanded by either a Colonel or Brigadier General accordingly. They always include one battalion of attack aircraft, one squadron of recon & attack aircraft, one company of long-endurance unmanned aerial systems(UAS), and at least one battalion of transport and assault aircraft. 
Other enablers include one battalion of air defense artillery, one engineering battalion, one signal battalion, and one military police company.
The sustainment command consists of one administrative company, one maintenance battalion, one supply & transport battalion, one medical battalion, three brigade support battalions, and one CBRN defense company. It also has administrative control over the artillery support battalion, aviation support battalion, and the forward support companies that sustain each fighting battalion.
Army divisions are very large, with authorized strengths of almost 20,000 soldiers. 
=== Light Infantry Division ===
Light infantry divisions primarily fight on foot, as well as using unarmored and lightly armored vehicles. Their logistical requirements are smaller than other unit types. They are by far the easiest unit type to transport by air and space, and they require less rolling stock than other types to transport over land. Light infantry are also capable of operating in severely restrictive terrain where heavier units could not maneuver. However, their tactical mobility is limited, since they do not organically have enough trucks or aircraft to move the entire division at once, and the maneuver units fight dismounted. Lack of heavy weapons also limits their combat power compared to heavier units. 
The maneuver component is divided into six light infantry battalions, three intermediate infantry battalions, and one divisional tank battalion, plus the light cavalry squadron. The light infantry units fight on foot and rely on soft-skinned vehicles for mobility, while the intermediate infantry are mounted in lightly armored trucks. The divisional tank battalion provides companies and platoons of tanks where armored support is necessary. One of the three brigade HHBNs uses lightly armored vehicles like the intermediate battalions, while the other two are light infantry. Per TOE, the default arrangement is to keep all three intermediate battalions in one brigade for ease of administration, while the other two brigades are purely light infantry. This is typically modified when going into combat. 
The divisional artillery's three brigade fires battalions use a mix of light towed howitzers, which can be lifted by utility aircraft, self-propelled howitzers, and small-caliber multiple-rocket launchers (MRLs). One battalion of lightweight MRLs and one battalion of heavier self-propelled howitzers provide longer-ranged fires for the division commander.
The aviation component includes the attack, attack/recon, and UAS assets common to all division types, plus two assault battalions instead of the one transport battalion found in heavier units. This enables a greater portion of the division to move by air at once, taking advantage of their light weight.
The air defense artillery and engineering battalions both utilize unarmored and a few lightly armored vehicles. 
The sustainment command follows the standard organizational lines, but the units within are slightly smaller than in other unit types. They have a greater portion of light transport trucks suitable for rough terrain.
==== Air Assault Division ====
Air Assault Divisions are a subtype of light division that make extensive use of vertical-lift aircraft for mobility. They require specialized training for deploying from aircraft. Air assault battalions have a reduced quantity of heavy & bulky equipment compared to light infantry battalions, and their divisional artillery omits heavy howitzers in favor of a second MRL battalion. They have a divisional weapons battalion in place of a divisional tank battalion, replacing some of the tanks with lighter platforms. Most importantly, air assault divisions have four assault aviation battalions instead of two. 
==== Other Subtypes ====
The UNSC Army employs various subtypes of light infantry division designed for use in specific terrain, such as tundra or extreme Karst regions. Such divisions usually differ from standard light infantry by using nonstandard vehicles and requiring specialized training.
=== Mechanized Division ===
Mechanized divisions move and fight using wheeled armored vehicles. They have larger logistical requirements than light units, but are less maintenance- and energy-intensive than armored units. They take up significant amounts of space shipping capacity to transport, and air transport of mechanized units is not feasible on a large scale. Since they possess enough wheeled vehicles for the entire division to move mounted, they can drive themselves long distances, making for excellent operational mobility. However, their tactical mobility is constrained by their vehicles' limited off-road capability. Having protected mobility for all maneuver forces and a moderate amount of heavy weapons affords significant combat power to mechanized units.
The maneuver component contains six mechanized infantry battalions and four intermediate infantry battalions, plus a mechanized cavalry squadron. The mechanized battalions use medium or heavier wheeled vehicles, in contrast to the lightly armored trucks of the intermediate battalions. One of the three brigade HHBNs uses lightly armored vehicles matching the intermediate battalions, while the other two use heavier vehicles matching the mechanized battalions. Per TOE, the default arrangement is to keep all four intermediate battalions in one brigade for ease of administration, while the other two brigades are entirely mechanized. This is typically modified when going into combat. 
The divisional artillery is entirely self-propelled, mostly based on trucks with light appliqué armor. The brigade fires battalions use medium self-propelled howitzers and small-caliber MRLs, while the divisional fires consist of one heavy howitzer battalion and one medium MRL battalion. 
The aviation component has one battalion each of attack, attack/recon, and transport, plus the standard UAS company. 
The air defense artillery and engineering battalions use a mix of light, medium, and some heavier armored vehicles. 
The sustainment command's battalions contain more maintenance and fueling assets in order to sustain the armored vehicles' needs. 
=== Armored Division ===
Armored divisions fight using tracked armored vehicles. These are maintenance intensive and consume lots of hydrogen fuel, creating the highest logistical requirements of any division. Armored units take up somewhat more shipping capacity for space transport, cannot be moved by air at any significant scale, and require rail or heavy truck transport for long marches. Some terrain, such as jungles or mountains, severely restricts their mobility. Yet their tracks provide excellent off-road mobility and allow for more heavily built vehicles than a mechanized unit. The combat power of an armored unit is unparalleled, since the entire fighting force is mounted in vehicles with medium or heavy armor and a large quantity of heavy weaponry. 
The maneuver component consists of ten combined arms battalions (CABs), which have either two tank companies and one infantry company (CAB-AR) or two infantry companies and one tank company (CAB-IN). Five CABs are CAB-INs, four are standard CAB-ARs, and one is a Heavy CAB-AR using heavy tanks in place of main battle tanks. All brigade HHBNs use tracked armored vehicles, as does the armored cavalry squadron. Per TOE, the default arrangement is for two brigades to each have one CAB-AR and two CAB-INs while the third brigade has two CAB-ARs and one CAB-IN, with the Heavy CAB-AR administered separately. 
The divisional artillery uses entirely tracked armored platforms. The brigade fires battalions use medium howitzers and small-caliber MRLs, while the divisional fires consist of one heavy howitzer battalion and one tracked MRL battalion. 
The aviation component has one battalion each of attack, attack/recon, and transport, plus the standard UAS company. 
The air defense artillery and engineering battalions use assorted tracked vehicles.
The sustainment command has more maintenance and fueling assets than any other. 
== Heritage ==
Like many UNSC Army units, Army divisions typically use the names, guidons, and some other traditions of long-established divisions, including many that predate the UNSC itself. Due to centuries of reorganization and reflagging, the actual link between a historical forebear and a current unit are often entirely arbitrary, but the traditions continue.