Ancient army numbers

[Sean Manning]

I will have to find this. And add it to my list of 'to reads'.

I still believe we can not totally discount the massive size of the invasion force.
What would be stopping the supply lines being cut, if there was not a garrison of the areas the forces moved through?

First off, I don't know of any evidence that there were land supply lines (in the sense of a continuous flow of supplies overland from Persian Thrace). Ancient armies didn't use huge detachments to hold conquered territory; Alexander and the Romans used a few thousand second-rate troops scattered widely, and messengers with small bodyguards.

Edit: Xerxes and Alexander also used hostages (you don't think that all those troops Alexander brought along from Thrace and Greece but hardly used were there for their fighting value do you? If anyone in Thrace had rebelled, Alexander had a lot of influential men he could kill in various interesting ways)