Ancient army numbers

[Gaius Julius Caesar]

Macedon does however explain quite clearly how it would be feasible for the army to have been quite large, and the Persians capable of raising it and supplying it.

Indeed he does! But to me that's also where it ends - in pure theory.
Of course it could be possible, humanly speaking, if you had the time, the power, the means.
But that is my point in all this (and of Roach also, I suspect) is the lack of hard evidence. I mean, no-one takes Herodotus literally here, but if we follow this through, what do we have?

Robert, the King of Kings was able to command the entire resources of the Empire. He had the time and the means.Preparation:

An operation of this size must have taken years to prepare. The logistics of it all must have been staggering. I mean, look at what was done before Normandy 1944! And that with only a tribal-based feudalist iron Age society, the pace with which such plans could have been implemented must have been a good deal slower than what the Allies needed – and that was months and months.

It is documented that it was years in the planning. In fact was it not 10 years between Marathon and Thermopolae?

Next, where do you put all those men? And how do you control all of them?
Suppose the larger part of the army would assemble in one or two spots, perhaps more, and march in separate columns towards Europe (I keep thinking of Napoleon’s and Hitler’s invasions of Russia) to keep a grip on them. But even in 3 or 4 groups, the strain on the countryside (friendly countryside) must have been enormous.
So do we have any indication where this could have taken place? Camps for hundreds of thousands of men?

They were assembling in the lands which lead up to the Hellespont I imagine, Sardis possilby being the command center?

The same goes for the fleets, which would have been essential for transport and supplies while the army was in Greece – I think everybody will agree with me that supply lines over land would have been to long and would have needed too much transport to even contemplate them as feasible.
Where would these fleets assemble? Or would we have to think of a great number of smaller fleets, each commanded separately?

Why not? The harbours of the western coast would only accomodate so many ships each.
There are many ports and beaches where fleets can muster.

For both land and sea forces, the same problem would occur: how do you control them? Persia was a feudalist state, I doubt very much that they would have been able to control each army and fleet from a central point, and I just do not see how an operation like this could have relied on commanders operating independently.

How was any pre-modern empire controlled?
Infrastructure

So I suppose that this was prepared in advance, by setting up storage facilities for food, water (especially water!), complete with live animals and their fodder.
Where are these facilities? There must have been dozens and dozens, each very large, for all the grain, non-perishable food, water, fodder and containment pens for the large herds of animals.

This is where many later armies failed. They could not get the supplies to the troops, even when organized far better than Xerxes. So how could he have managed where they all failed?

He was as successful as all of them, no more. But, his enemy was not of a size tha tthe ywere able to immdiately persure his withdrawing armies.

Is there any evidence for storage facilities where the fleets could have picked up their loads? I mean, this was so huge (and see my point on command) that it’s almost impossible to expect each city, island and region to act independently as a supply base.

Invasion