Armies grew to huge sizes from the 1880s onwards, numbering in the hundreds of thousands, and in the case of the First World War, millions. Supported by growing populations and huge industries, and equipped with rifles, artillery and machine guns that greatly increased the firepower available to Generals, things like 'campaign seasons' and 'winter quarters' essentially became obsolete; fighting could be maintained for months, and ultimately years, on end, requiring multiple campaigns, huge battles, and massive casualties before one side could hope to over come the other; the Western and Eastern Fronts in both wars are a case in point.

At the same time, the size of armies outstripped the available communications technology, and the ability of individual generals to command 'great hosts' as in the past, leading to increasingly larger staffs and a greater chain of command, often involving dozens of corps, multiple armies and army groups. 'Battlefields' like the Somme took up areas the size of London and the Metropolitan area south of the Thames, or in the case of Stalingrad, whole, sprawling cities. Generals ceased to be 'Great Captains' per se, and more like Managers, issuing directives to subordinates who then followed through on their commander's intent.

All of this was essentially the result of the Second Industrial Revolution that swept the world in the 1880s and 1890s. For more on the technological/firepower side, and the implications thereof, [I'd recommend this excellent lecture by Nicholas Murray.] (https://www.youtube.com/watch?v=2cbq7iu8FrI)