So how's about we discuss the possibility that, in an ATL where Cyrus the Younger defeats his brother Artaxerxes II, the 10,000 Greek mercenaries he brings perform so well that he decides to award them with land in Mesopotamia.
This leads to a not insignificant state of Sparta-influenced warriors in Mesopotamia that can possibly make or break kings, and a stronger Achaemenid Persia (Cyrus was a warlike go-getter, whereas OTL's winner was Artaxerxes, who focused too much on the pleasures of rule, ignoring the duties).
Is this a possibility/would Cyrus allow the Greeks to settle in Mesopotamia?
As for the "Sparta-influenced warriors":
- at this period of time the commanders ("sergeants and officers") of any mercenary Greek force were usually Spartans or Athenians as the most "noble" among the Hellenes. That was a tradition and that was quite natural.
As for settling in Mesopotamia:
1) why settling the Greeks if the Empire might have them as a standing army, on a payroll? The Achaemenids had huge reserves of gold and they definitely could afford that.
2) why in Mesopotamia? They might be distributed along the borders and/or in the most rebellious provinces, where surrounded by the dangerous enemies they would be less inclined to rebell themselves.
As for the Greeks making and breaking kings:
- Cyrus was a natural-born Achaemenid Prince. The essence of the Achaemenid Empire was the rule of the Persians all over the Empire. The Greeks might have been instrumental in making him a king and possibly for a few months to consolidate his power.
But soon, very soon they would become a nuisance, even a shame for him. Worst possible scenario is the Greeks are massacred by the Persians. Most probable - the Greek 10 000 are showered with lavish rewards and sent home. One thing for sure - Cyrus would use the Greeks, not these Greeks who won him the throne but other guys, as mercenaries and military settlers all over his Empire on much larger scale than before - quite traditionally as the Persians had used the Jews and other non-Persian peoples. A few thousand Greeks as the core of the Grand Army crack force infantry of shahanshah is possible. But the dominance of the Persians in the Empire and in the army would be indisputable.