Eli5. Why do military groups (101st airborne for example) seem to be numbered in random order
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Unit numbers are generally sequential. Often, they reserve certain ranges of numbers for a certain role (such as for reservists or for for a different type of unit or something). If an army shrinks and has to cut those units, it’s probably going to try to keep the more prestigious ones.
Let’s take your example of the 101st Airborne Division.
In the numbering convention of the United States Army, division numbers 1-25 were active duty, 26-75 were National Guard, and anything above that was Army Reserve. Unlike some other armies, the United States doesn’t number armor divisions or cavalry divisions in the same sequence, so right now if the United States Army brought its heavy armor to bear all on one front, you would see 1st Infantry Division, 1st Cavalry Division, and 1st Armored Division rolling in with their tanks and other heavy tracked vehicles, though currently there are no other divisions in either the armor sequence or the cavalry sequence.
The Army Reserve doesn’t have combat arms troops anymore, but a few of those numbers are in use for other purposes such as 100th Infantry running certain training units. National Guard still has a few divisions such as 29 and 42. The last armor division other than 1st Armored was 49th Armored in the Texas Army National Guard.
82nd Airborne and 101st Airborne were originally Army Reserve infantry divisions created during the World War I expansion of the Army. 82 saw combat (the most decorated American of the war, Alvin York, was. In this division), but the war ended before 101 made it to Europe and it was a reserve unit until those two divisions were organized as parachute divisions. After the war when the Army was downsizing and returning the higher-numbered divisions to reserve status, they kept 82 and 101 as active-duty divisions because the name is famous and draws lineage to people who the Army likes to talk about and the 101st keeps the airborne name even though they don’t jump out of planes anymore (although when 101 was reconstituted during Vietnam as an air assault division riding helicopters into battle, it was originally as the 11th Airborne Division).
Since 1945, the United States Army has never been as large as it was during the war and some of the divisions during the war had been fake divisions to fool German spies even if they existed in the World War I Army, there are a lot of gaps in the number sequence.
Sometimes, the Army brings back divisions. Most recently, the 25th Infantry Division’s two brigades in Alaska were reorganized as the 11th Airborne Division (the only WWII Pacific parachute division) so 25ID is now just the guys in Hawaii. Around 10-15 years ago, they did the same thing and revived 7th Infantry Division out of the 2nd Infantry Division brigades in the Continental United States, so 2ID is now only the units in Korea.
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