Why did Greek mercenaries become important to the later Achaemenid Empire?

In the later years of the Achaemenid Empire, Greek mercenaries became increasingly important as the empire faced significant internal instability, including frequent rebellions, dynastic power struggles, and the loss of territories like Egypt.12

The reliance on these soldiers was driven by several key factors:

• Military Utility: Greek hoplites were considered the most effective fighters in the Mediterranean, and their specialized phalanx formation provided a tactical advantage that the Persian military sought to utilize.34
• Internal Conflict: Persian kings and rebellious satraps alike hired Greek mercenaries to bolster their forces during civil wars and attempts to suppress uprisings. For example, Cyrus the Younger famously employed 10,000 Greek mercenaries in his failed bid for the throne at the Battle of Cunaxa.51
• Campaigning Needs: The Persian state frequently used Greek mercenaries to support their repeated, often difficult, military campaigns to recover control over Egypt.51
• Financial Leverage: The Achaemenids possessed significant wealth, which they used to pay for these troops, effectively turning the mercenaries into a professional force whose loyalty was secured through financial compensation rather than political allegiance.512