Cyrus advances unopposed into Babylonia, but Artaxerxes, warned at the last moment by Tissaphernes, is hastily gathering an army.
The decisive battle is fought at Cunaxa, on the left bank of the Euphrates River north of Babylon, where the two armies meet unexpectedly on September 3.
Cyrus's Greek mercenaries seem initially to have won the battle, routing the Persian left with few casualties, while Cyrus himself charges Artaxerxes' center with six hundred cavalry.
However, Cyrus loses his life in trying personally to kill his brother, whom he only wounds, at which point the mercenary army falls apart.
The Greeks fight on fiercely through the day, but eventually surrender the fighting ground and begin a retreat. (The Athenian mercenary Xenophon will accord Cyrus high praise in the “Anabasis,” his lively firsthand account of the retreat of the Ten Thousand).
The Greeks return to find that the rest of Cyrus' troops have been routed and his camp plundered.
The Persians, during truce negotiations with Tissaphernes, who has supported Artaxerxes and distinguished himself in the battle, treacherously seize Clearchus and other Greek leaders, then turn them over to Artaxerxes, who executes them.
The defeated remnants of the Ten Thousand, trailed by Persian troops, make a desperate thousand-mile retreat from Babylonia through the heart of hostile territory toward the nearest friendly haven, the Greek colony of Trapezus on the Black Sea coast.
The Greeks, led by Xenophon and others during the five-month march, live off the land, repel attacks from barbaric hill tribes, and suffer through a harsh winter.
