Our heavy divisions make sense. We have a cavalry division that we call the 1st Cavalry Division, an armored division called the 1st Armored Division, and four mechanized infantry divisions which are, sensibly enough, called the 1st, 2nd, 3rd and 4th Infantry Divisions.
But the light divisions baffle me. Why do we have the 82nd Airborne Division, when we don’t have 81 other airborne divisions, or indeed 81 other divisions of any kind? Or why is our air assault division called the “101st Air Assault Division”? I don’t see how it could help confuse the enemy, since it’s pretty well a matter of public record that we have only ten full-time active-duty divisions.
The numbering of divisions started with WWI. Before that there were sometimes divisions (organizations of two or three brigades ) but they were not permanent organizations. The First and Second Divisions, and maybe the Third Division were regular army outfits. The First Division was initially two Army brigades and one brigade of Marines. The other divisions were numbered as they came on line. The same sort of thing happened in WWII. The 82 and the 101 Divisions were simply the 82d and 101st divisions orginized, but after the war there was an interest in maintaining the same sort of linage at the division level that had long existed with the regiments. Thus you had the 101 Airborne Infantry Division when the army clearly did not have 101 infantry divisions, let alone 101 parachute divisions.
During Vietnam there was some name changing to take advantage of the unit history thing. Thus the 11th Infantry Division, which had been extensively trained in helicopter tactics became the First Cavalry Division. I’m not sure what happened to the soldiers who were in the First Cav the day before the name change.
During World War II there were series of divisional numbers reserved for divisions formed from national guard units and as such there were a fair number of unused numbers - I think there was a 117th infantry division formed but we never fielded 117 divisions of all types.
They weren’t far off, though. The US Army had 77 infantry and airborne divisions in World War II, and 16 armoured divisions, and I remember reading that there were plans for several hundred divisions had the war dragged on. Those units that remain now are those divisions that remained from downsizing.
Spavined, I believe what would have happened is this: the sub-divisional units of the two preexisting divisions that would not fit in the “reorganized” division would have been attached to other commands, or disbanded and their personnel reassigned; those that did would be renumbered and much of the personnel making up the reorganized 1st. Cav would be mustered as “new” sub-divisional units. Considering this was the time of the draftee Army you’d have many privates and junior NCOs leaving at the end of each year anyway, so you’d just have to relocate career officers and NCOs; sometime in the 50s the “regimental” system had been changed to purely ceremonial so a career soldier was no longer committed to a specific unit, but could be sent to any open billet he fit. AFTER Vietnam the “Air Cavalry” mission was given to the 101st AB, and the 1st. Cav became Armored cavalry. After that,l the sub-units of the 1st. Cav. themselves gradually were “conformed” to traditional cavalry regimental numberings.
Sometimes, a number will be retired and then revived for historic or political reasons. The 65th Infantry Regiment, for instance, an all-Puertorrican (xcept for the senior officers) unit from before WW2, was dissolved after the Korean War. But decades later the Puerto Rico National Guard lobbied to revive the “65th” designation for a Separate Infantry Batallion of the Guard, using a regimental crest similar to the original 65th, in part to make historic esprit a motivator for recruiting, retention and motivation; in part to honor the veterans of the original 65th, which doesn’t hurt politically.
The same kind of thing happens elsewhere as militaries get smaller. The British Army’s 7 Armoured Brigade, currently in Iraq, uses the same insignia and claims the same history as the famous 7th (“Desert Rats”) Armoured Division that fought in North Africa in World War II.
Veteran here. I was a member of the 6th Infantry Division (Light) back in the “cold war”, one of the “Light Infantry” divisions and at the time the sister division to the 10th Mountain. The 6th ID is now decommissioned. It had been created and decommissioned and re-created multiple times since WWI. None of the instantiations of the 6th ID had anything to do with each other other than the designation and some of the heraldry.
1917: initial organization of the 6th ID; sent to France during the war; returned to Illinois after the war
1921: deactivated
1939: reactivated in Washington state and then Missouri; entered war in South Pacific
1949: deactivated
1950: reactivated for Korean War
1956: deactivated
1967: reactivated for Vietnam War
1968: deactivated
1986: reactivated in Alaska, now as Light Infantry
1994: deactivated
This sort of thing is fairly common. Units are activated and deactivated, and designations are re-used, and the units really don’t have much in common except the name, the heraldry, and the history.