Do you know the difference between a battalion and a brigade, between a regiment and a divisin? If not, the thousand words or so below will help you. It is as clear as anything I have seen. (via Volokh Conspiracy).

MILITARY COMMAND JARGON: A TENTATIVE PRIMER. Which is the bigger unit: the 101st Airborne or the Seventh Marines? That’s an easy one: the 101st Airborne is much bigger; it’s a division, whereas the “Seventh Marines” refers to a regiment. I feel obliged to mention this because I just heard a television commentator refer to the “Seventh Marine Division” -- a blunder. It occurred to me that it might be useful to post a general summary of the sizes and commanders of the different infantry units in the Army and Marines. The usefulness will be twofold. In the event, however unlikely, that you aren’t at all clear on these points already, they may increase your understanding of the coverage of the war. If you are clear on these points already, you may know more about them than I do and be able to correct any mistakes that exist in what follows; I will amend it accordingly. I’m no expert. I know most of what little I do because my father was a Marine.

A "fire team" consists of a few men commanded by a corporal.

A squad consists of about twelve men (three teams) commanded by a sergeant – typically a total of thirteen soldiers.

A platoon’s size can vary, but it typically consists of three squads, or 39 soldiers, plus a platoon sergeant and a platoon leader -- a lieutenant.

A company consists of two or more platoons (usually three or four) -- typically between 150-200 soldiers -- and is run by a captain.

A battalion consists of two or more companies (e.g., three "rifle" companies plus a heavy weapons company) -- perhaps 700-800 soldiers -- and is run by a lieutenant colonel. Players of Stratego may be wondering at this point what a Major does. A major typically is an executive officer -- second in command -- in a battalion. Or he can run the battalion if lieutenant colonels are in short supply, or command a company if they are short of captains.

A regiment typically consists of three or four battalions and is run by a full colonel: around 3,000-5,000 soldiers.

A brigade typically consists of a couple of regiments, but also can be smaller than the numbers that formulation would suggest. They traditionally have been run by brigadier generals, but now in the Army brigades are commanded by full colonels. A reader informs me that the Army evidently no longer uses "regiments" as fighting units. It’s brigades instead. (You can go here for some further explanation; you may have to scroll down, as I couldn’t get the permalink to work.) The Marines, by contrast, only form brigades for particular and occasional purposes. Brigades and regiments are largely interchangeable for purposes of comprehending news coverage. The important thing is to distinguish them from divisions.

A division typically consists of three infantry regiments plus more: an artillery regiment, supply staff, intelligence staff, logistics staff, headquarters staff, motor transport people, a medical battalion, and some other headings, ending up with 15,000-25,000 or so soldiers -- 18,000 is the rule of thumb, but a paratrooper battalion could be attached, or a tank regiment, etc., with a resulting variation in numbers. It is run by a major general (a "two star" general). There also are distinct armored divisions; these differ because they consist mostly of tanks and other armor that support the infantry divisions. The Marine Expeditionary Force is a unit roughly on par with a division in its scale, though larger as it includes its own air support and other amenities.

A corps consists of two or more divisions (maybe 60,000 soldiers) and is run by a three-star general -- a lieutenant general. V Corps ("Fifth Corps") is the outfit in Iraq now. It is run by William Wallace.

A group of "corps" make up an "army" or a command -- in the case of Iraq, the central command -- which is run by a full (four-star) general. (There are no five-star generals now; Eisenhower, Bradley, and a couple of others from WWII had that distinction.) In this case the general is Tommy Franks, who oversees not only the V Corps but also the Air Force and Naval units there, etc.

Examples of usage:

The Seventh Armored Brigade (the "desert rats") is a British unit, a "brigade" because it’s too small to be considered a "division." "Brigades" are more common in British usage than American.

The "101st Airborne" (the "screaming eagles") is a division of the Army that traditionally consisted of paratroopers (plus support of various kinds), but now is a more general air assault division making extensive use of helicopters, etc. This division fought famously for Bastogne in the Battle of the Bulge during WWII. Many of the divisions and regiments you hear about now have storied histories, having participated in great battles of prior wars. It can be fun to go do a search for a regiment’s name; usually there are sources on the web providing some history.

Divisions go by numbers, as do regiments and battalions. So within the First Marine Division, there are infantry regiments that that have numbers of their own -- the first marine regiment, the fifth, and the seventh. An ambiguity can arise here that is related to the television miscue that precipitated this post: someone can refer to the "First Marines." This should not mean the first division; it should mean the first *regiment* (which is in the first division). The first division is better referred to as the First Marine Division. This is the division that went into Guadalcanal; it was the first bunch of Marines to fight in WWII (at least in an offensive capacity; thus I am setting aside the Marine detachment at Wake Island, which a reader reasonably reminded me not to overlook).

If someone refers to "the 31st infantry," on the other hand, this refers to a regiment, not a division. The resulting rule of thumb: when you refer to a division, use the word "division." If you refer just to a number -- the 31st infantry, or the 7th marines, or the 3d cavalry -- you are referring to a regiment. Yes, the “101st Airborne” is an exception; it’s a division. That’s why only rules of thumb are possible.