Ancient army numbers

[Sean Manning]

The Greek sources claim that Darius III marched 600,000 (plus supply animals) through the Bache Pass to Issus. Reduced to (a generous) four abreast and a metre of spacing through the mountains, this army will have stretched 150 kilometres (not counting supply animals. And this will have been similar wherever supply wagons and such demanded roads to move on.

Indeed. We very rarely see anything like the Napoleonic practice of dividing armies into corps which marched separately but came together briefly to fight, in the ancient world. This is what briefly made it practical to gather a hundred thousand or more men in one place, until population growth and high explosives forced armies to spread out again in the 20th century. Herodotus says that Xerxes' armament gathered in Cappadocia and wintered inland at Sardis, so clearly it could survive for months without waterborn supply or splitting up.