With sufficient resources, a player can train up to 100 Legions without any Villagers in the Iron Age fresh from the Barracks without surpassing the population limit. This technology is exceptionally useful to civilizations with infantry bonuses such as the Choson and the Romans.
The science of supplying armies on the march was called logistics. The Assyrians were the first great army to employ logistics consistently, and this allowed them to operate regularly at great distances from their home bases. Armies on campaign needed food, forage for animals, equipment, armor, and weapons. In most cases, food was acquired from the countryside through which the army was moving. But if an army paused to besiege a town, it exhausted local supplies quickly. Food and forage had to be brought in or the army was forced to retire. The tools of logistics were supply wagons, pack animals, depots, and troops to keep the roads open. The great conquerors of antiquity mastered logistics.