IIRC there's a long series on the acoup blog referenced elsewhere in this discussion, that deals with the logistics of moving and feeding an army in pre-modern

IIRC there's a long series on the acoup blog referenced elsewhere in this discussion, that deals with the logistics of moving and feeding an army in pre-modern times. Tl;dr: When you're limited to beasts and wagons, the possible size of your army is severely curtailed. Also foraging for food (again, with beasts and wagons there's only so much food you can transport over any non trivial distance) puts a quite low limit on the size of an army you can have in an area. IOW, the modern mass army enabled by conscription is also dependent on a modern logistics system to supply them.