Persian Invasion of 480 BC - articles

[Aryaman2]

Here's what I would like to know. In order for there to be a significant logistical difference between a field army and the global army, some aspect of this endeavor would not scale linearly with the others and act as a limiting factor, so which is it? It's kindof a muddled question, let me give an example:

If I can supply 100 guys marching for 3 days with a supply dump of 900 pounds of grain, I should be able to supply 100,000 guys with 900,000 pounds. If 100 guys leave 300 pounds of poop on their nightly stops, 100,000 would leave 300,000 pounds of poop. If 100 guys needs 300 gallons of water every day, then 100,000 would need 300,000 gallons a day.

Something must scale differently to prevent this army from increasing in size indefinately. I suspect it can't be just one thing, but maybe a few together. What are this things that cannot simply be scaled up to maintain any size army?

First of all it depends of the supply system the army is using.
If it is basically getting supplies by foraging, then the limit is of course the land capacity to provide supplies, in ancient times those supplies are almost exclusively food. For example, an army of 2.000 could live off the land without having to dispatch detachments to get food to more distant places, while an army of 10.000 would have to dispatch those detachments, or even splitting the army, limiting the forces available for battle, of course the numbers that could be fed without splitting an army would vary according to different conditions, like weather, season, terrain, composition of the force (cavalry is much more demanding than infantry). In any case a field army using foraging as his main supply system has to be rather small.
If the army is using either supply depots or a fully developed supply lines system, the main limitating factor is transport capabilities. Soldiers can carry 5-7 days rations by themselves, the rest has to be transported. Land transport until the developments of railways remained slow and costly, the cost increasing exponentially with the size of the force, since the draw animals also had to be fed. Besides that there is traffic jams, there was a limit to the number of wagons that could go down a given road without collapsing it, and of course with dirty roads things could go much worse in rainy seasons.
The march rate could be dictated by supply shortages, for instance XVIII century armies stopped regularly every 3 days so that flour could be converted in bread in campaign ovens, so that some empty wagons could be sent back to supply depots.
To put an example, a field army advancing along a road had the supply train marching behind, an army resting in a crossroad that could be supplied by different roads could be larger, an army placed in a port could be supplied by sea/river lanes, so it could become even larger, but the moment it moved away from the city down a road, it could be supplied only along that road, so armies ususally split to take different roads "march apart, fight together" was the rule, armies marched by different routes trying to converge on certain strategic places to face the enemy. That is why so many times we read of reinforcements arriving in the middle of a battle.
Some historians have tried to apply mathematical formulae to supply limits, but there are too many factors, you can estimate a maximum easily, but not a minimum as too many things could go wrong.
For modern armies, of course, supply limits are more obvious, as they consume large quantities of fuel and ammunition as well as food.