Bit of a sidenote, but I've been listening to Dan Carlins podcast re. WWI.
And he says that at the height of the Roman Empire they had somewhere between 250k and 400k in the army to protect the entire realm. Sources obviously vary, but it's somewhere in that ball-park.
Now zoom back to the 20th century, and Germany had close to 1M men going through Belgium in August '14, just for the flanking move.....
Yes granted, the populations are much higher in the 20th century, but still it does illustrate why you can't run a modern army without some serious logistical support.
A million men means a million gallons of drinking water each day, plus food, plus fodder for the draught-horses, plus ammunition, plus replacements, plus....etc. Ad Nauseum.