Terminology

Unit sizes 

Fireteam  
2–4 men. Commanded by a Lance Corporal, or Corporal.
A fireteam or fire team is a small military subunit of infantry designed to optimize "bounding overwatch" and "fire and movement" tactical doctrine in combat. Depending on mission requirements, a typical fireteam consists of four or fewer members; an automatic rifleman, an assistant automatic rifleman, a rifleman, and a designated team leader. Two or three fireteams are organized into a squad or section in coordinated operations, which is led by a squad leader.


Squad/ Section
8–14 men. Commanded by a Corporal, a Sergeant, or a Staff Sergeant.
A squad or squadron is a subunit led by a non-commissioned officer that is subordinate to an infantry platoon. In countries following the British Army tradition (Australian Army, Canadian Army, and others), this organization is referred to as a section. In most armies, a squad consists of eight to fourteen soldiers, and may be further subdivided into fireteams.


Platoon/ Troop  
15–45 men. Commanded by a Second Lieutenant, a First Lieutenant or a Lieutenant.
A platoon is a military unit typically composed of two or more infantry squads/sections/patrols. There are other types of infantry platoons (e.g., antiarmor, heavy machinegun, light armored reconnaissance, mortar, reconnaissance, scout, scout sniper, and weapons), depending upon service and type of infantry company/battalion to which the platoon is assigned, and these platoons may range from as few as 18 to 69. Non-infantry platoons may range from as small as a nine-man communications platoon to a 102-man maintenance platoon infantry/combined arms battalion). A platoon leader or commander is the officer in command of a platoon. This person is usually a junior officer—a second or first lieutenant or an equivalent rank. The officer is usually assisted by a platoon sergeant. A platoon is typically the smallest military unit led by a commissioned officer. Rifle platoons normally consist of a small platoon headquarters and three or four sections.


Company/ Battery/ Squadron
80–150 men. Commanded by a Captain or a Major.
Most companies are formed of three to six platoons, although the exact number may vary by country, unit type, and structure. Usually several companies are grouped as a battalion or regiment, the latter of which is sometimes formed by several battalions. Occasionally, independent or separate companies are organized for special purposes, such as the 1st Air Naval Gunfire Liaison Company or the 3rd Force Reconnaissance Company. These companies are not organic to a battalion or regiment, but rather report directly to a higher level organization such as a Marine Expeditionary Force headquarter.

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Examples of battalion sizes; an Allied tank destroyer battalion 
and a German heavy tank battalion (503, in 1944)

Battalion/ Cohort
300–800 men. Commanded by a Lieutenant Colonel.
A battalion is the smallest military organization capable of "limited independent operations", in that the battalion is the lowest-level organizational unit containing an organic coordinating, or executive, staff and a support and services unit (e.g., headquarters and headquarters company). The battalion must have a source of re-supply to enable it to sustain operations for more than a few days. This is because a battalion's basic load of ammunition, expendable weapons (e.g., hand grenades and disposable rocket launchers), water, rations, fuel, lubricants, replacement parts, batteries, and medical supplies normally consists of only what the battalion's soldiers and the battalion's organic vehicles can carry.

In addition to consisting of sufficient personnel and equipment (usually at least two primary mission companies and one mission support company) to perform significant operations, as well as a limited self-contained administrative and logistics capability, the commander is provided with a full-time staff whose function is to coordinate current operations and plan future operations. A battalion's subordinate units (companies and their organic platoons) are dependent upon the battalion headquarters for command, control, communications, and intelligence and the battalion's organic service and support structure to perform their mission. The battalion is usually part of a regiment, brigade, or group, depending on the organizational model used by that service.


Regiment/ Brigade/ Legion
1,000–5,500 men. Commanded by a Colonel, or a Brigadier General.
The word "regiment" – much like "corps" – may have two somewhat divergent meanings, which refer to two distinct roles: a front-line military formation; or an administrative or ceremonial unit.

In the regimental system, each regiment is responsible for recruiting, training, and administration; each regiment is permanently maintained and therefore the regiment will develop its unique esprit de corps because of its unitary history, traditions, recruitment, and function. Usually, the regiment is responsible for recruiting and administering all of a soldier's military career. Depending upon the country, regiments can be either combat units or administrative units or both.

Some regiments recruited from specific geographical areas, and usually incorporated the place name into the regimental name. In other cases, regiments would recruit from a given age group within a nation (e.g. Zulu Impis), an ethnic group (e.g. the Gurkhas), or foreigners (e.g. the French Foreign Legion). In other cases, new regiments were raised for new functions within an army; e.g. the Fusiliers, the Parachute Regiment (British Army), and the U.S. Army 75th Ranger Regiment.

A brigade is a major tactical military formation that is typically composed of three to six battalions plus supporting elements. It is roughly equivalent to an enlarged or reinforced regiment. Two or more brigades may constitute a division.

Brigades formed into divisions are usually infantry or armored (sometimes referred to as combined arms brigades). In addition to combat units, they may include combat support units or sub-units, such as artillery and engineers, and logistic units or sub-units. Historically, such brigades have sometimes been called brigade-groups. On operations, a brigade may comprise both organic elements and attached elements, including some temporarily attached for a specific task.

Brigades may also be specialized and comprise battalions of a single branch, for example cavalry, mechanized, armored, artillery, air defence, aviation, engineers, signals or logistic. Some brigades are classified as independent or separate and operate independently from the traditional division structure.

A brigade's commander is commonly a major general, brigadier general, brigadier or colonel. In some armies, the commander is rated as a General Officer. The brigade commander has a self-contained headquarters and staff. The principal staff officer, usually a lieutenant colonel or colonel, may be designated chief of staff, although until the late 20th century British and similar armies called the position 'brigade-major'. Some brigades may also have a deputy commander. The headquarters has a nucleus of staff officers and support (clerks, assistants and drivers) that can vary in size depending on the type of brigade. On operations, additional specialist elements may be attached. The headquarters will usually have its own communications unit.

In some gendarmerie forces, brigades are the basic-level organizational unit.


Division
10,000–25,000 men. Commanded by a Major General.
In most armies, a division is composed of several regiments or brigades; in turn, several divisions typically make up a corps. Historically, the division has been the default combined arms unit capable of independent operations. Smaller combined arms units, such as the American regimental combat team (RCT) during World War II, were used when conditions favored them. In recent times, Modern Western militaries have begun adopting the smaller brigade combat team (similar to the RCT) as the default combined arms unit, with the division they belong to being less important.


Corps
30,000–50,000 men. Commanded by a Lieutenant General
A corps may be: an operational formation, sometimes known as a field corps, which consists of two or more divisions, or an administrative corps – that is a specialized branch of a military service (such as an artillery corps, a medical corps, or a force of military police) or in some cases, a distinct service within a national military (such as the United States Marine Corps). These usages often overlap.

In many armies, a corps is a battlefield formation composed of two or more divisions, and typically commanded by a lieutenant general. During World War I and World War II, due to the large scale of combat, multiple corps were combined into armies which then formed into army groups. In Western armies with numbered corps, the number is often indicated in Roman numerals (e.g., VII Corps).


Field Army
100,000–300,000 men. Commanded by a General.
A field army is a military formation composed of two or more corps and may be subordinate to an army group. Likewise, air armies are equivalent formation within some air forces. Particular field armies are usually named or numbered to distinguish them from "army" in the sense of an entire national land military force. In English, the typical style for naming field armies is word numbers, such as "First Army"; whereas corps are usually distinguished by Roman numerals (e.g. I Corps) and subordinate formations with ordinal numbers (e.g. 1st Division). A field army may be given a geographical name in addition to or as an alternative to a numerical name, such as the British Army of the Rhine, Army of the Niemen or Aegean Army (also known as the Fourth Army).

Field Marshal Bernard Law Montgomery

Army Group/ Front
2+ field armies. Commanded by a Field Marshal or a Five-star General.
An army group is a military organization consisting of several field armies, which is self-sufficient for indefinite periods. It is usually responsible for a particular geographic area. An army group is the largest field organization handled by a single commander—usually a full general or field marshal—and it generally includes between 400,000 and 1,000,000 soldiers.

Army groups may be multi-national formations. For example, during World War II, the Southern Group of Armies (also known as the U.S. 6th Army Group) comprised the U.S. Seventh Army and the French First Army; the 21st Army Group comprised the British Second Army, the Canadian First Army and the US Ninth Army.

In both Commonwealth and U.S. usage, the number of an army group is expressed in Arabic numerals (e.g., "12th Army Group"), while the number of a field army is spelled out (e.g., "Third Army").


Region/ Theater
4+ army groups. Commanded by a Six-star rank offiver or a Commander-in-chief.
Military districts (also called military regions) are formations of a state's armed forces (often of the Army) which are responsible for a certain area of territory. They are often more responsible for administrative than operational matters, and in countries with conscript forces, often handle parts of the conscription cycle.

Navies have also used a similar model, with organizations such as the United States Naval Districts. A number of navies in South America used naval districts at various points in time.

In warfare, a theater or theatre (see spelling differences) is an area or place in which important military events occur or are progressing. A theater can include the entirety of the airspace, land and sea area that is or that may potentially become involved in war operations.