Military rank
Rank and unit size

To get a sense of the practical meaning of these ranks—and thus to be able to compare them across the different armed services, different nations, and the variations of titles and insignia—an understanding of the relative levels and sizes of each command is helpful. The ranking and command system used by U.S. Marine ground forces or U.S. Army infantry units can serve as a template for this purpose. Different countries will often use their own systems that do not match the presentation here.

Under this system, starting from the bottom and working up, a corporal leads a fireteam consisting of three other individuals. A sergeant leads a squad consisting of three fireteams. As a result, a full squad numbers 13 individuals. Squads usually have numbered designations (e.g., 1st Squad).

Generally, in most armies and marine units, a lieutenant or equivalent rank leads a platoon, which can consist of three or four squads. For example, in U.S. Marine infantry units, rifle platoons usually consist of three rifle squads of 13 men each, with a Navy corpsman, the platoon leader, and a platoon sergeant (i.e., a staff sergeant who serves as second-in-command). An infantry platoon can number from 42 to 55 individuals, depending on the service. Platoons are usually numbered (e.g., 1st Platoon) or named after their primary function (e.g., service platoon).

A captain or equivalent rank commands a company, usually consisting of four platoons (three line platoons and one heavy weapons platoon). His headquarters can include a first sergeant and as many as seven others. As such, a company can comprise from roughly 175 to 225 individuals. Equivalent units also commanded by captains are batteries (for field artillery units) and detachments. In English speaking countries, a company (or troop in the cavalry, and battery in the artillery) is usually designated by a letter (e.g., "A Company"). In non-English speaking countries, they are usually numbered. In most Commonwealth armies a company is commanded by a major, assisted by a captain.

A lieutenant colonel or equivalent rank commands a battalion or a squadron, often consisting of four companies, plus the various members of his headquarters. A battalion is around 500–1,500 men and usually consists of between two and six companies.

A colonel or equivalent commands a regiment or group, often consisting of two or more battalions (for an infantry unit) or five to six air groups (for a wing). Battalions and regiments are usually numbered, either as a separate battalion or as part of a regimental structure (e.g., 1-501st Infantry in the US Army).

The next level has been a brigade, commanded by a brigadier general, and containing two or more regiments or three to eight battalions. In the U.S. Army, a brigade is roughly equal to or a little larger than a regiment, consisting of three to seven battalions. Strength typically ranges from 1,500 to 3,500 personnel. In the U.S. Marines, brigades are only formed for certain missions. In size and nature they are larger and more varied collections of battalions than is common for a regiment, fitting them for their traditional role as the smallest formation able to operate independently on a battlefield without external logistical tactical support. Brigades are usually numbered (e.g., 2nd Brigade).

The level above regiment and brigade is the division, commanded by a major general and consisting of from 10,000 to 20,000 persons. The 1st Marine Division, for example, is made up of four marine regiments (of the type described above), one assault amphibian battalion, one reconnaissance battalion, two light armored reconnaissance battalions, one combat engineer battalion, one tank battalion, and one headquarters battalion—totalling more than 19,000 marines. (Within the headquarters battalion are one headquarters company, one service company, one military police company, one communications company, and one truck company.) An equivalent elsewhere within the same marine expeditionary force (MEF) might be an MEF logistics group (MLG), which is not a regimental-sized unit (as the word "group" implies), but rather a large support unit consisting of several battalions of support personnel. Divisions are normally numbered, but can be named after a function or personage.

The command sizes for any given rank will vary widely. Not all units are as troop intensive as infantry forces need to be. Tank and artillery crews, for example, involve far fewer personnel. Numbers also differ for non-combat units such as quartermasters, cooks, and hospital staff. Beyond this, in any real situation, not all units will be at full strength and there will be various attachments and detachments of assorted specialists woven throughout the system.

In the U.S. Army, the level above division is a corps. It is commanded by a lieutenant general. In many armies, a corps numbers around 60,000, usually divided into three divisions. Corps (and similar organizations) are normally designated with Roman numerals and their nationality when operating in a combined (international) force—e.g., V (US) Corps, VIII (ROK) Corps, II MEF, I Canadian Corps.

During World War II, due to the large scale of combat, multiple corps were combined into field armies commanded in theory by a general (four stars), but often by a lieutenant general (three stars), and comprising as many as 240,000 troops. Armies are numbered by spelled-out numerals or functional titles, using their nationality in "combined" forces (e.g., Eighth United States Army, Third ROK Army, British Army of the Rhine). These were in their turn formed into army groups, these being the largest field organization handled by a single commander in modern warfare. Army groups included between 400,000 and 1,500,000 troops. Army groups received Arabic numeral designations and national designations when combined- e.g. 21st Army Group.
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