I am amazed at the quote - the author must have access to some hitherto unknown tally of the army of Xerxes!

I’m reading Sea Power and it’s says “Civilization in Greece was at stake.

Let's be clear - the Persians were not benevolent enlightened despots. They threw their weight around by employing overwhelming military strength. But the claim that "civilization" was at stake is patently absurd. If anything, Classical Greek civilization as we think of it was a product of interaction with Near Eastern and Iranian cultures and ideas. The Achaemenid state apparatus controlled by any measure the most advanced and powerful political entity that had ever existed.

Moreover, as can be seen in Ionia, they permitted self-rule as long as tribute was paid.

The key, in the end, was freedom. Each of the Greeks was a free man

Except, uh, all the slaves? Herodotos maintains there were seven Helots for each Spartan at Plataea.

, fighting for his family and his city-sate. Virtually all of the Persians were conscripts or slaves.”

That's a nice conflation, "conscripts or slaves". We know little about the army of Xerxes, but we can surmise from Herodotos that it was archery-heavy and contained both lightly and heavily armed troops. The heavy Iranian troops, including the royal guard and Medean contingents, were probably drafted from a martial elite. Lighter troops would have been conscripted from Syria and Asia Minor.

Slavery in the Achaemenean realms generally is a complex matter. Corvee or conscripted labour for e.g. construction work was definitely a thing at least in Mesopotamia and Persia proper. Mostly, language we have does not distinguish well between terms like slave, labourer, minister, servant, etc. The Greek perception that all people were slaves under the Great King is linked to this - in Near Eastern royal ideology, subject peoples are frequently depicted as fettered to the king, and loyal ministers described in terms akin to "bond-servant". Forced labour of various kinds probably existed throughout the realms (only Mesopotamia and Persia are all that well documented). But there is no reason to think there was the kind of all-encompassing slave society we see in Greece. The Persepolis tablets suggest the exisrence of a thriving caste of professional workers and craftspersons aserving the royal estates and administration, including, interestingly, highly compensated women workers supervising large labour groups. We also have mentions of certain satrapies providing labourers as tribute.

Finally: Slavery had long been declining under Assyrian dominion, and given the copious amounts of tribute the Great King raised, I'm not sure employing much of anything but fairly compensated professionals would have made sense - though it is hard to imagine war captives not being put to work. But there's no evidence of a Rome/Greece-like "slave economy".

This claim manages to be wrong on multiple levels.

One, using the loaded term "conscript" to describe Achaemenid levies but not Greek levies misrepresents what these militaries were. In both the Achaemenid Empire and the Greek city-states there was a certain portion of the population who had military obligations and who were subject to conscription. Greek citizens were obligated to serve as soldiers in a capacity commensurate with their wealth and social position- there was nothing voluntary about their service. So in describing one side as "conscripts" and the other as "free men," the author is stating his preference for Greek political systems and not actually describing two different ways of making men into soldiers.

Two, the claim that "slaves" made up a significant part of the Achaemenid army seems to lack any basis. Slavery took different forms in different parts of the Persian Empire, but slave-soldiers are not attested (for comparison, Greeks also practiced slavery on a considerable scale, and did not draft slaves to fight either). The belief that the Achaemenids commanded an "army of slaves" comes from the Greek perception that all subjects of the King of Kings were, at least theoretically, his slaves- but these people did not actually have the legal status of slaves and many of them, like those serving in the cavalry or the Immortals, were men of high status.

Three, the idea that the Persians threatened to destroy "civilization" in Greece is, again, a mix of hyperbole and loaded language. The Greeks of Asia Minor had already been under Achaemenid rule for decades, without losing their cultural or political institutions. Likewise there was no "end of civilization" in Macedonia or the Greek cities of Thrace after they offered up earth and water. What was at stake was not the "civilization" of Greece but the full political autonomy of its communities. The Greek poleis and koina did eventually lose this autonomy, first to the Macedonians and then finally to the Romans- yet we don't regard this as the end of their civilization.