Army Unit Numerical Designations: Where do they come from?

If you’re like me, you’ve always wondered where some of the wonky unit numbers in the US Army come from. Like, where did the other 99 airborne divisions other than the 82nd and 101st go? Did we ever have 503 infantry regiments at the same time?

The answer comes down as a combination of some incredibly prosaic rule-making with Army force structure as well as some completely arbitrary nonsense. Basically, peak Army. 

We begin, of course, around the time of World War I. Because, as I point out to anyone who wants to listen, that’s the era that changed the US Army. And one of the most profound changes that came in that time was the establishment of permanent divisions as combined arms units.

Before the Great War

Now we’d had divisions before. Prior to this time, divisions and corps tended to be merely organizational features in time of war. Regular Army regiments and U.S. Volunteer regiments would be organized into respective brigades, divisions, and corps as part of a field army. So you’d have, say, the Fifth Corps, Army of the Potomac. Inside the Fifth Corps would be the First, Second, and Third Divisions. Inside the First Division would be the First, Second, and Third Brigades. So, a Soldier in the 16th Michigan Volunteer Infantry Regiment would say that he was in the Third Brigade, First Division, Fifth Corps, Army of the Potomac. Phew. That’s a mouthful.

This type of organization existed for a few reasons. Namely, we didn’t have a big standing Army and we weren’t really relying on the Regular Army in time of war. The Army was a constabulary force. During times of war, Congress authorized the President to call up U.S. Volunteers from the states. Oftentimes these were established militia units which would enter the service in a group. So for example, the 6th Massachusetts Regiment of Volunteer Militia, in existence since 1855, entered service in 1861 as the 6th Massachusetts Volunteer Infantry Regiment. Most of the time, however, volunteer units were really just that – volunteers filling spaces in state units going in federal service.

First Attempts at Professionalization

With the Spanish-American War in 1898, the Army realized that something needed to change. The mobilization of U.S. Volunteer units was filled with as much chaos and disorganization as the war itself – which is saying a lot, because 1898 was a dumpster fire. Reforms began coming aimed at modernizing and streamlining the Army’s force structure. In 1903, the Militia Act authorized a robust National Guard that would be modeled off of the Regular Army. While it removed more autonomy from the states, the Militia Act provided more funding and incentives to the National Guard.

The War Department attempted some permanent force structure initiatives prior to 1916, but they were…interesting. In 1909, Assistant Secretary of War Robert Oliver divided the nation into eight military districts, a mix of Regulars and National Guard – although participation from the Guard was voluntary, and so therefore, limited.

But even by 1911, the Army was still wrangling over what the primary tactical unit would be. A corps or field army, or a division? The arguments and writing of Andrew A Humphreys won out; remarkable, since he’d been dead since 1883. 

We have to take a brief digression to Humphreys because, oh my word, what a mind. Born in 1810 to a Navy family – his grandfather was Joshua Humphreys, that absolute genius who designed the original six frigates of the U.S. Navy, including USS Constitution – Humphreys took a left turn and went to the U.S. Military Academy at West Point. Graduating as an engineer, he quickly racked up scientific achievements. When the Civil War rolled around, Humpheys then distinguished himself as a good leader and also as a man of “distinguished and brilliant profanity.” He finished the war as a corps commander and retired from the Army in 1879, when he went on to study philosophy and get an honorary degree from Harvard. So, the guy kinda had a decent brain.

Humphreys had argued for a strong combined arms division to be the basic tactical Army organization, designed to fight on its own. One of his reasons was experience of terrible roads in the U.S., and he argued that corps organizations were too large to try to mass at one point. So the Army decided to go for a strong division.

But the 1910 and 1911 organizations floundered and eventually fell apart. Secretary of War Henry L. Stimson tried again in 1912, with the Stimson Plan – another very originally named plan. By 1914, Stimson had battled his way to four Regular Army infantry divisions – 1st to 4th Divisions – and had managed to get the National Guard to – mostly – agree to twelve infantry divisions, number 5th-17th.

A Permanent Force Structure

But it was 1916 where the big changes came. The National Defense Act of 1916 – imaginative name, I know – came as a product of six months of work by the General Staff followed by wrangling in Congress. The output was the organization of permanent tactical divisions, for the Regular Army and the National Guard. So that was all settled.

Psych, no, it wasn’t, because just as everyone was about to make all these organizational changes in 1917, the U.S. enters World War I. And what France and the Entente want right away is some sort of visible example of the U.S. commitment to the war. So the War Department throws together an expeditionary division, consisting of four infantry regiments and one field artillery regiment. Labeled the “1st Expeditionary Division” it is the 1st U.S. Division (the “infantry” designation not entering until the 1930s). 

Meanwhile, back at the Army War College, they are trying to figure out how the hell they’re going to figure out numerical designations for all the Regular Army, National Guard, and National Army – units formed out of selective service draftees – units. They come up with the following plan:

Divisions numbered 1-25 are Regular Army. 26-75 are National Guard. 76 and above, National Army. At the time, each infantry division contained four infantry regiments, held in two brigades, and three artillery regiments, in one field artillery brigade. Numbers for brigades ran from 1-50 for the Regular Army, 51-150 for the National Guard, and 150 and above for the National Army.

Naturally, this changed the old National Guard division numbers of 5-17, so they began the conversion. New England’s 5th Division became the 26th, New York’s 6th Division became the 27th, Pennsylvania’s 7th Division became the 28th, and so on around the country. 

At the same time, it became necessary to end the old state designations of units. This was often painful, as the state designations had powerful meaning to the members. To make it even worse, some units were entirely broken up to fill vacancies in the division since they were organizing for full war strength. Flags of old organizations such as the 1st New Hampshire and 1st Vermont were folded and placed in storage as the new Federal designations took hold.

As with divisions and brigades, regiments were given blocks of numbers as well. The Regular Army regiments retained numbers 1-100. Thus, in the 26th Division, the first National Guard division to arrive in France in 1917, the infantry regiments were the 101st, 102nd, 103rd, and 104th. The field artillery regiments were similarly 101st, 102nd, and 103rd, supported by the 101st Engineer Regiment, 101st Sanitary Regiment and…you get the picture. In the National Army, the numbering began at 301 and went to infinity and beyond. Like the National Guard, the National Army was organized by region, beginning in New England, working its way down the east coast, south, midwest, and ending in the west. 

Interwar and World War II

Following World War I, the National Army concept disappeared as a component of the Army of the United States. It was replaced with the Organized Reserve, today’s Army Reserve. The “square” infantry division centered around four infantry regiments in two brigades disappeared in 1941, as the Army decided that the three maneuver regiments per division were good enough. This arrangement caused another organizational shuffle as units were redesignated and scooted around. But by and large, the numbering system enacted in WWI stayed in place for each component during World War II, with a few exceptions.

The two most notable were the 82nd and 101st Airborne Divisions. At the outset of WWII, the War Department took the 82nd and 101st Infantry Divisions, which had been National Army in WWI and Organized Reserve in the interwar period, and made them airborne divisions, along with the newly formed 11th, 13th, and 17th. Similarly, the 10th Mountain Division had begun as an infantry division in WWI, was given the moniker “light” at the beginning of WWII, and then became “mountain.” There were no 1st-9th Mountain Divisions. Armor and Cavalry Divisions similarly followed the 1917 numbering convention. 

In 1946, as the Army downsized again, one of the quandaries became how to perpetuate the lineage of units that had overgone so many changes, especially with the redesign of the divisions. The decision was made to perpetuate the history of units in the geographic area where they were from, for the National Guard, and to a certain extent the Organized Reserve. This trend would continue through the 1960s and ‘70s as more and more Regular, Guard, and Reserve divisions were inactivated. 

The Weird Times: 1950s through 1960s

In 1957, things got weird. The Army started playing with atomic weapons and strange ideas like single-seater helicopters that Soldiers could fly around on like a cavalry charge. They also scrapped regiments as tactical units. The building block for the Army would be the battalion, as it continues to be today. In order to save the old regimental lineages, the Army adopted the Combat Arms Regimental System (CARS). Regiments would cease to exist, but their designation would continue through parent units run by the Department of the Army to perpetuate lineage and honors. This was – and is – done by the Army Center of Military History.

It was at this time that non-divisional regimental combat teams disappeared and were replaced by combined arms brigades – the brigade reentering the force structure after a brief hiatus. Indeed, through the ‘60s, the brigade crept deep into the force structure, replacing some divisional elements, especially in the National Guard and Reserve. Most – but not all – numbering for brigades attempted to adhere to historical units. For example, the 43rd Infantry Division was inactivated in New England, partially replaced with an infantry brigade from Vermont and Connecticut, given the designation of the 86th after the pre-WWII 86th Infantry Brigade, part of the 43rd Infantry Division.

 Between 1959 and 1961, all three components of the Army underwent some serious organizational trauma. Units changed designations and organizations as many as three or four times. 1959 also saw the transition of many of the Army Reserve divisions to training, as thirteen in total converted from combat to training divisions.

In 1965, the Army cut the last five combat divisions of the Army Reserve. The purpose of this was to enhance the manning of the three infantry divisions and six brigades of the National Guard that consisted of the Selected Reserve Force (SRF). This force was to report to their mobilization stations within seven days of a call-up. They were authorized to be fully manned, received more drill days, and were prioritized for new equipment. Sounds eerily familiar to focused readiness and Guard 4.0.

So yeah, Army Reserve conspiracy theorists, eat your hearts out: Secretary of Defense Robert McNamara sacrificed your combat divisions for greater National Guard readiness. 

1968 saw the Regular Army get all huffy when they made the 101st Airborne Division a helicopter unit and tried to change its name. In a compromise to history and pissed-off officers, both the 101st Airborne and 1st Cavalry Divisions were allowed to keep their designations, even though they no longer applied. The Army also axed most of the Army Reserve’s armor units, citing poor manning. Y’all see a trend here?

The Readiness Decade: the 1970s

Following Vietnam in the 1970s, more brigades and divisions were inactivated across all components in order to create higher readiness levels for the units that remained. By 1974, the Regular Army had thirteen divisions and four brigades, the National Guard was left with eight divisions and eighteen brigades, while the Army Reserve was down to just three brigades. Four years later, the number of divisions was upped to twenty-four. To meet this requirement, the Department of the Army authorized the National Guard and Reserve to provide round-out brigades to Regular Army divisions. Sort of like…the associated units program. 

All things old are new again.

The 1980s saw the heavy hand of reorganization hit the Regular Army, with some divisions redesignated from heavy to light and the activation of the 10th Mountain Division (which had to have a brigade stationed at Benning because Drum didn’t have capacity for more than one brigade). It also saw more and more military intelligence units going to the Army Reserve. Nearly all MI units in National Guard divisions were Army Reserve, due to the need for special equipment and storage areas for classified information. 

Post-Cold War Changes

Following the fall of the Soviet Union and Operations Desert Storm/Shield, the Army began a draw-down. The last few Reserve brigades were inactivated as well as several Regular Army and National Guard divisions. The brigade combat team replaced the division as the tactical unit of the Army as the division headquarters took on a command and control role. Many National Guard divisions had their lineage continued in brigade combat teams (37th IBCT in Ohio, for example, perpetuates the 37th “Buckeye” Division) or maneuver enhancement brigades (26th MEB perpetuates the 26th “Yankee” Division of New England). The Army Reserve lost all of its combat units save for the 100th Battalion, 442nd Infantry.

So sad, Army Reserve; so sad. But hey, they somehow managed to get most of the civil affairs units, which I hear are almost special forces, so that’s cool. 


The bottom line here is that there was a general idea for unit numbering at the outset of all of this force modernization stuff in 1917. That has generally been adhered to, although at the company and detachment level it’s mostly gone to hell. However, there’s at least an attempt to keep some order in the chaos. For example, units that have been inactivated in the National Guard cannot have their numeric designations stolen by the other components, and most states have a bank of historical numbers to use as well. 

Regular and Reserve units have lineages that are governed mainly by the paperwork of the Army. For the National Guard, they continue to be tied to the geographic area from where they recruit. Unit identities going back hundreds of years can thus be carried on through the present, even though the units bear no similarity to their forebears.

The one certainty is that the Army changes, often. But hanging on to things like unit lineages and even the specific numbering of units can help connect us with our past. It is in looking to our past that we can help set our minds at ease concerning the not-insignificant force structure changes that lie ahead of us.


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About the Author: Angry Staff Officer is an Army engineer officer who is adrift in a sea of doctrine and staff operations and uses writing as a means to retain his sanity. He also collaborates on a podcast with Adin Dobkin entitled War Stories, which examines key moments in the history of warfare.

Cover Photo:  2nd Infantry Division marching up the bluff at the E-1 draw in the Easy Red sector of Omaha Beach on D+1, 7 June 1944. They are going past the German bunker, Widerstandsnest 65, that defended the route up the Ruquet Valley to Saint-Laurent-sur-Mer. US Army Signal Corps Photo.

3 Replies to “Army Unit Numerical Designations: Where do they come from?”

  1. I always loved reading the lineage of a unit and seeing where it said the unit moved from one place to another — especially when I was in the unit and I know that the only things that moved were the guideons and name. At least the more recent lineage documents now usually say the unit was “inactivated” at one place and “reactivated” at another place, which is less misleading.


  2. This is a wonderful short history of American Army lineages. I especially enjoy the snarky asides that come from someone who must be living through restructuring and the constant influences of the bright ideas guys who always want to rebrand or reinvent how we do business as a military.

    I look forward to reading more of your insights.

    Liked by 1 person

  3. The WWI brigade lineage were kind of sorta incorporated into the current division lineage over the years. The lone exception was the 2nd Brigade of the 2nd Infantry division which was created from scratch in the late 1950s. the second brigade within the 2nd Division in WWI was the 4th Brigade, but was a US Marine Corps unit.

    The current battalions of regiments, delineate from the regimental companies (1st Battalion 25th Infantry in delineated from A company 25th Infantry). There pecking order for which units remain on active duty is determined by adding up the number of years of service and the number of campaign streamers and unit awards.


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