The Persian imperial army was mostly not ethnically Persian. This is analogous to the situation in the Roman empire. Augustus reduced the size of the army after he prevailed in the civil war with Antony, cutting it roughly by half to 28 legions (plus the Praetorian Guard, which originally was selected from the best of the best in the army), but then supplementing Roman forces with 28 sort-of legions recruited from conquered peoples, and used in situations where their tactical specialties were needed.
Anyway, the Persian armies Alexander defeated had a lot of Greek mercenaries, as well as Greek soldiers born in various Persian provinces (such as those in Anatolia, but also in Syria, Palestine, Phoenicia, Egypt, etc) serving in regular units of the Persian army. After he whipped everyone he faced, he took Persepolis and seized the massive treasury. The Persian king (or by that time, it may have been the pretender, I can’t remember when the Persians themselves whacked Darius) had fled, and no longer had the cash to pay his army. Also, the Greeks in Anatolia and points east were (mostly) grateful for their liberation, and of course the soldiers needed the work.
Very interesting, thanks.