The Immortals

[Paralus]

[Diodorus' army strengths in that area of his History look plausible IIRC, especially if you interpret those ten thousand Persians as "a division with a paper strength of 10,000." Eumenes had been Alexander's secretary, and Diodorus' source was probably Eumenes' secretary: they knew logistics and the strength of the regular parts of their own army.

Sean is on the right track with his line of argument here. Diodorus' source here is Hieronymus who took part in this campaign on the side of Eumenes. He later took service under Antigonus.

Firstly, the Helleno-centric view of the Persian army is to be viewed with caution. Xenophon’s Cyropaedia is often a fictionalised account of history. If one were to take the lexicon of Greek writing on the Achaemenid Empire and, more importantly – its military forces – at face value, one would need to seriously question how the Persians ever came to conquer an empire at all. The Helleno-centric argument is, well, they only beat Asians, they couldn’t beat Greek hoplites. All of which begs the question of the conquest of the Ionian Greeks.

Much of this is wrapped in the usual confectionary of Greeks defeating thoroughly ridiculous numbers of Persians (such as Alexander at Issus and Gaugamela). That the Persians conquered the old kingdoms of the east (Media, Babylonia and the trans-Euphrates) as well as Bactria in the time of Cyrus without effective infantry is not likely. Nor is it possible that a “small numberâ€