But in most movies about the twentieth century wars, it always seems divisions are most important. Was this a real change, or just a change in the way it's presented in writing? Was it because of the increased size of the Armies?
Also, if it doesn't violate rule 2 (or just talk about the 1990s), a what unit level do military personal identify in the last twenty-five years? (Does a transfer overseas mean they lose their unit identity forever.)
I guess this is mostly about the US military, because it's so big, but I won't mind hearing about other countries.
Until after the Napoleonic Era, and indeed MUCH later in some armies(such as the US) anything above the Regiment was a temporary organization "in the field" as it were. Basically as a function of the number of Regiments/Battalions assigned to an army and often left up to the Commander on the spot to organize as he saw best.
In the US Army you dont really see anything larger than a division until the Civil War. General Scott's army in Mexico for instance was just a grouping of divisions organized as the army was gathered. It wasnt even until the Spring of 1862 either when Lincoln ordered that Corps be formed to better manage the Army of the Potomac both in battle and administratively. Running into the same problems that prompted the French to adopt the system in the first place 60 years before, that the jump from a 10k man Division to a 100k man Army is too big and awkward and there needs to be a middle layer.