It all began with sibling rivalry. Darius II (r. 424–404 bc), Great King of Achaemenid Persia, had many children with his wife Parysatis, but his two eldest sons Arses and Cyrus got the most attention. Parysatis always liked Cyrus, the younger of the two, better. Darius, though, kept Arses close, perhaps grooming him for the succession. Cyrus he sent west to Ionia on the shores of the Aegean Sea, appointing him regional overlord. Just sixteen when he arrived at his new capital of Sardis, the young prince found western Asia Minor an unruly frontier. Its satraps (provincial governors), cunning and ruthless men named Tissaphernes and Pharnabazus, often pursued virtually independent foreign policies, and sometimes clashed with each other. There were also western barbarians for Cyrus to deal with. Athens and Sparta, now in the twenty-third year of their struggle for domination over Greece (today we call it the Peloponnesian War, 431–404 bc), had brought their fleets and troops to Ionia. The Athenians needed to preserve the vital grain supply route from the Black Sea via Ionia to Athens; the Spartans wanted to cut it.

The Achaemenids had their own interest in this war: after two humiliatingly unsuccessful invasions of Hellas in the early fifth century, they wanted to see Greeks lose. Hoping to wear both sides down, the western satraps had intermittently supported Athens and Sparta, but Darius desired a more consistent policy. That was one reason why Cyrus was in Ionia, to coordinate Persian efforts.