@Trav'ling Canuck: Good summary, with just a few additions/comments:
From Antiquity to 1815 CE there is a continuity in size of battlefield, place and actions of the Commander, and concentration of men on the battlefield. The formations of Waterloo in 1815 were not appreciably different from those of Gaugamela in 333 BCE: infantry in dense lines, cavalry maneuvering in 500 - 1000 man units, both armies laid out in dense linear formations.
The first big change came with the introduction of gunpowder weapons, which firmly moved the Killing Power to Ranged Weapons instead of Melee, and started the process of thinning the lines and the density of the formations. It was now paramount that all the men carrying muskets got a chance to use them, so deep formations wasted firepower and were gradually reduced to the "Thin Red Line" - which was still shoulder to shoulder, but only 2 ranks deep instead of 6, 10, or 16 ranks as in ancient, classical, or medieval armies. The fact that the army was drawn up in 2 or more lines, however, meant the battlefield was still pretty densely packed and all that gunpowder had a lot of targets.
The next big change was in Command and Control. Up until the middle of the 18th century, there was virtually no permanent organization between the brigade of 2 - 4000 men and the Army of 50,000 +. Army Commanders had 'Wing' Commanders (the successor to the medieval 'Battle') but they were organized on the battlefield and frequently Wing Commanders had nothing in the way of staff or support to exercise real control. It was not until after 1750 CE that first France, then Russia introduced Divisions of several brigades to produce an intermediate command structure to make it easier to control large armies. By the early 19th century (1805, to be exact), Napoleon introduced the Corps d'Armee (Army Corps) or several infantry divisions supported by batteries of artillery and brigades of cavalry that formed 'little armies' that could operate independently or, together, control really huge armies by all previous standards.
It was not the machinegun, but the indirect fire artillery (recoil mechanisms and advanced trigonometry) firing huge amounts of High Explosive that caused armies to 'spread out' dramatically. The armies of the Franco-Prussian War, the last major war between Great Powers without such weapons, covered about the same space per man as the armies of Napoleon, even though, supposedly, the rifles and artillery had much greater range. The problem wasn't range, it was finding a target through all the gunpowder smoke and controlling armies of 100,000 men or more if they spread out too far: couriers on foot or on horseback were moving at the same speed they had for Alexander at Gaugamela or Napoleon at Waterloo. Both the armies of 1870 and the Russian and Japanese armies in Manchuria in 1905, which were 2 - 3 times larger, ran into serious problems trying to control, or even find out, what was happening on the 'battlefield'.
That problem got even worse in WWI. Not only were the battles even more spread out, but the near-universal mass of High Explosive being fired by everyone killed most communications: telephone wires broken, couriers blown to fragments, carrier pigeons shot down in droves.
The apparently increased mobility of the battles of WWII owed as much to radio as to the tank or motorized vehicle. It was radio communications while moving and down to individual companies or platoons that made it possible to (barely) control WWII battles, and to provide the support to successful small actions that turned them into breakthroughs, pursuits, and decisive victories.
In game terms,
Humankind seems to be using the
Endless Legend system of increasing the number of units allowed in an 'Army' in tactical actions as you progress through the Eras and allowing more 'off the battlefield' support later in the game to represent Reserves intervening or long range artillery/air support to the men in the 'killing field'. In game terms, I'd still like to see increases in concentration directly tied to certain Techs rather than simply by Era and the use of Reserves and other 'off the tactical map' intervention directly tied to Communications Technology - first the telephone/telegraph, then, especially, the radio and possibly a final Boost from Satellite Communications in the past 30 years which make electronic communications practically Universal down to the individual vehicle and soldier - a development with distinctly negative as well as positive aspects.
By the way, supporting your original thesis, in miniatures wargaming, the WRG rules cover battles from Ancient to Renaissance (pre-gunpowder) armies and 'ancients' tournaments at wargame conventions regularly pit armies as historically different as Incan or New Kingdom Egyptian and Teutonic Knights against each other. Because, of course, the general configuration of the units and armies is similar, regardless of their (all muscle-powered) weapons and armor.
Rich Hasenauer's
Fire and Fury rules for the American Civil War, one of the most popular sets of miniature wargames rules ever written, has spawned variations covering battles from 1701 CE (War of the Spanish Succession) to the Franco-Prussian War (1870 CE) with no major changes to the rules (Lots and lots of changes to the firepower factors, of course). BUT Rich's set of World War Two rules,
Battlefront, although based on the same game mechanisms, is so different from
Fire and Fury as to be almost unrecognizeable.
