My apologies to the moderator that some of the examples here cross over the 1900 barrier, but my offered discussion is definitely pre-1900.
Ever notice the difference in the battlefields between – say 1914 or 1944 and 1704 or 1863? Not just the weapons or even the tactics – just the density that came from mass armies?
In 1704, Marlborough could march an army from the Low Country to the Danube without having to mount the continual reconnaissance and assaults that Patton or Montgomery had to when crossing the same territory in 1944. J.E.B. Stuart could go off into Pennsylvania scouting for Federals without banging his head against the succession of defensive lines the German advance toward Paris encountered in 1914.
The density of troops on the ground were markedly different, but so was the system of control. Before the age of railroads and truck transport the armies were limited in size and deployment by the area of farmland that could support them. Marlborough and his contemporaries could handle an army of 100,000 because they could communicate with and supply that number of units within their control radius. Give them an army of a million, such as those engaged at Verdun, and they would have bogged down with the sheer task of controlling them.
Staff work you will say. Sure, but also the telegraph, the telephone and the radio. Mass armies were recruited before such inventions – Napoleon had them – but not without his band of Marshals commanding the multitude of spread out Army Corps. There were times when the Napoleonic Wars had campaigns that somewhat resembled the slogging matches of 20th Century conflicts, but they were the exception.
Having the mass army was only possible with the communications that could pass intelligence inward to the commander and orders outward to the troops. Would Marlborough have been able to handle an army group of a million spread across the Low Countries if he’d had radio? Would Lee have been able to advance on Washington with several armies in concert if he’d had the communications to coordinate the campaign?
What do you think?