To give you an example in Iberia, most Western and Northern tribes relied on animal husbandry rather than agriculture simply because the terrain was too mountainous and on the few plains the soil was either too hard or too...bleh, don't know the word in english. Something like sandy or rocky. Those are natural pasture lands which can't be changed to productive farming because farming in those lands was unproductive, whereas autotrophic plants grew normally. The same thing can be applied to areas of Persia, I suppose. So in those days, we couldn't really terramorph lands, but I'm not aware of the specifics of Persian Geography anyway.
...Why?
Who says they couldn't do both? I don't know what the first cost refers to, but the second cost would certainly be bigger, but not gigantically bigger then if they weren't nobles. Nobles didn't eat at banquets
everyday while on campaign. I'm sure they could have some more refined meals, but the cost of feeding them to any other human shouldn't be that far off. Still this "Cost of feeding armies" is a very subjective thing.
But they did hire nomads (I think), and these nomads would fight for the standing army and get payed. If so, what is the point? It certainly wasn't cheap to defeat a nomad invasion.
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