With the [Ionian] burning of Sardis in mind we may turn to the question of the Persian military presence in western Asia Minor. The evidence from the Greek sources is scrappy, but we know of a number of cases of fiefs being granted and of Persians having taken over good land. It is generally in rich plain land that we discover Persian landowners, in the Maeander valley, in the Colophon plain, the Hermus plain (Buruncuk), and the Caicus plain. Between these last two plains is the rough mountain country of Southern Aeolis, where we are told that the King’s writ did not run on the 460s when Themistocles went into hiding in one of the Greek cities there; and when the Persians took over the Maeander plain after the fall of Miletus (494 B.C.) the mountain country to the south was handed over to the intransigent herdsmen of the Halicarnassus peninsula. We also know that to the north, in the region of Mt. Ida, there were cities that had not been subjugated by the Persians until well on in Darius I’s reign [520-484 BCE]; and the impression that we receive is that the Persians occupied the good land without troubling themselves unduly over the control of the unproductive hill country, which was not suitable for fiefs.
J.M. Cook in Ilya Gershevitch ed. The Cambridge History of Iran Volume 2: The Median and Achaemenian Periods p. 253, 254
On the state of the empire, what emerges from Xenophon’s fine story [ie. the “Anabasis” set in 401 and 400 BCE], … is the fact that all the hill tribes, many of which appear in Herodotus’ list of contingents marching under Xerxes, were now out of control … This, and the weakness of the empire’s hordes of conscript infantry, was not lost upon the Greeks.
A.R. Burn in Cambridge History of Iran Volume 2: The Median and Achaemenian Periods p. 354
Specialists in the Achaemenid empire like to argue whether the kings and governors were losing control of the Achaemenid empire in its last century of existence, or whether the empire was always a bit ragged about the edges. Greek and Latin literature implies that the empire was falling apart, but the Greek and Latin literature which we have was written or chosen by people who ‘knew’ that the Persians had become decadent and been overthrown by Alexander. As the above quotes show, two distinguished British historians writing in the same year could interpret the same evidence in opposite ways.
Further Reading: One key term is the “Great Satraps’ Revolt”