Time was it was based on the major command. The 1st Regiment would have the 11th, 12th, and 13th Battalions, for example. As time wore on and units entered combat, it was decided to try and keep the intact the lineage of those that did especially well during reorganizations. Here's an example: Say that the 1st Regiment had the three battalions aforementioned. Some time later, it is decided that regiments will have only 2 battalions. If the 11th and 13th Battalions never won battle honors, but the 12th did, the 12th would be assured to stay active while on of the other two would have their colors cased and put on the shelf. Units that are formed later in the service history will have numbers assigned sequentially if they are new units or assigned based on the parent unit's available numbers (remember, some are historical and cannot be used in the same parent unit) if they are being restored to a parent unit.

Unit numbers are assigned in a hierarchical manor. They can be said in many ways though. Crazyhorse troop, 1st squadron, 3rd Armored Cavalry regiment, 3rd corp, 3rd Army.

"QUESTION: Why did the United States send the 82nd Airborne to defend Saudi Arabia, rather than sending, for example, the First Airborne, or the Second Airborne? ANSWER: The main thing you need to know about the Army is that the smallest organized unit is the fire team. Then comes the squad, the platoon, the company, the battalion, the brigade, the division, the corps, the Army, the Army group, and, finally, the theater of operations.The "82nd Airborne" is a division - about 15,000 men and women, by present standards. (A division in World War I was 28,000 soldiers.) The shocking, appalling news is that the numbers are kind of nonsensical. If the Army wants to parachute soldiers into a combat zone, it has no choice but to use the 82nd Airborne Division, because that's the only parachute assault division we have. The only other airborne division is the 101st, which uses helicopters. Those two divisions, together with the 24th Infantry, make up the 18th Airborne Corps. These numbers are deceptive, because the entire Army has only 18 active divisions (and remember, a division is smaller than a corps). The numbering system began in 1917 when the United States entered World War I. The idea was that divisions 1 to 25 would be permanent Army divisions, 26 through 50 would be National Guard, and 51 and higher would be "national Army" divisions specially raised to fight in World War I. Story continues below Over the years, some numbers were retired, just like a ballplayer's, and a few high-numbered World War I divisions became part of the permanent regular Army, which is why the 82nd is still around long after the kaiser lost his war. To make things more confusing there are three divisions that claim the number 1: First Infantry Division (Mechanized), First Armored Division, and First Cavalry Division. ("Cavalry" doesn't mean they still ride horses; in Vietnam they used helicopters and since 1976 they use tanks - just like the First Armored Division! The term "cavalry" is kept for sentimental reasons.) It would be possible to simplify all this by returning to sequential numbers. But that would only make it easier for the accursed enemy to keep track of us. Better to be baffling. "It's a matter of deception," says John Wilson, historian for the U.S. Army Center of Military History. It's also a matter of history. There's something about the number of a military unit that mists the eye. As Hemingway wrote in "A Farewell to Arms," "Abstract words such as glory, honor, courage, or hallow were obscene beside the concrete names of villages, the numbers of roads, the names of rivers, the numbers of regiments and the dates." " Source and further information: http://archive.deseretnews.com/archive/print/120753/MILITARY-UNIT-NUMBERS-ARE-NONSENSICAL.html You can explore the "Military units and formations of the United States" on those pages: http://en.wikipedia.org/wiki/Category:Military_units_and_formations_of_the_United_States http://en.wikipedia.org/wiki/Category:Airborne_divisions_of_the_United_States_Army http://en.wikipedia.org/wiki/Category:Armored_divisions_of_the_United_States_Army http://en.wikipedia.org/wiki/Category:Infantry_divisions_of_the_United_States_Army http://en.wikipedia.org/wiki/Divisions_of_the_United_States_Army