He was born after the accession of his father in 424 BC. When, after the victories of Alcibiades, Darius II decided to continue the war against Athens and give strong support to the Spartans, he sent in 408 the young prince into Asia Minor, as satrap of Lydia and Phrygia Major with Cappadocia, and commander of the Persian troops, "which gather into the field of Castolos", i.e. of the army of the district of Asia Minor.
He gave strenuous support to the Spartans; evidently he had already then formed the design, in which he was supported by his mother, of gaining the throne for himself after the death of his father; he pretended to have stronger claims to it than his elder brother Artaxerxes II (Artaxšacā), who was not born in the purple. For this plan he hoped to gain the assistance of Sparta. In the Spartan general Lysander he found a man who was willing to help him, as Lysander himself hoped to become absolute ruler of Greece by the aid of the Persian prince. So Cyrus put all his means at the disposal of Lysander in the Peloponnesian War, but denied them to his successor Callicratidas; by exerting his influence in Sparta, he brought it about that after the battle of Arginusae Lysander was sent out a second time as the real commander (though under a nominal chief) of the Spartan fleet in 405.
At the same time Darius fell ill and called his son to his deathbed; Cyrus handed over all his treasures to Lysander and went to Susa. After the accession of Artaxerxes II in 404, Tissaphernes (Ciθrafarna) denounced the plans of Cyrus against his brother but, by the intercession of Parysatis, he was pardoned and sent back to his satrapy.
Meanwhile Lysander had won the battle of Aegospotami and Sparta was supreme in the Greek world. Cyrus managed very cleverly to gather a large army by beginning a quarrel with Tissaphernes, satrap of Caria, about the Ionian towns; he also pretended to prepare an expedition against the Pisidians, a mountainous tribe in the Taurus, which was never obedient to the Empire.
Although the dominant position of Lysander had been broken in 403 by King Pausanias, the Spartan government gave him all the support which was possible without going into open war against the king; it caused a partisan of Lysander, Clearchus, condemned to death on account of atrocious crimes which he had committed as governor of Byzantium, to gather an army of mercenaries on the Thracian Chersonesus, and in Thessaly Menon, head of a party which was connected with Sparta, collected another army.
In the spring of 401, Cyrus united all his forces into the group now called the "Ten Thousand" and advanced from Sardis, without announcing the object of his expedition. By dexterous management and large promises he overcame the scruples of the Greek troops against the length and danger of the war; a Spartan fleet of thirty-five triremes sent to Cilicia opened the passes of the Amanus into Syria and conveyed to him a Spartan detachment of 700 men under Cheirisophus.
The king had only been warned at the last moment by Tissaphernes and gathered an army in all haste; Cyrus advanced into Babylonia, before he met with an enemy. Here ensued, in October 401, the battle of Cunaxa. Cyrus had 10,400 Greek hoplites and 2500 peltasts, and besides an Asiatic army under the command of Ariaeus, for which Xenophon gives the absurd number of 100,000 men; the army of Artaxerxes he puts down at 900,000. In reality the army of Cyrus may at the very utmost have consisted of 30,000, and that of Artaxerxes, 40,000 men.
Cyrus saw that the decision depended on the fate of the king; he therefore wanted Clearchus, the commander of the Greeks, to take the centre against Artaxerxes. But Clearchus, out of arrogance, disobeyed. As a result the left wing of the Persians under Tissaphernes was free to engage the rest of Cyrus' forces; Cyrus in the centre threw himself upon Artaxerxes, but was slain in a desperate struggle. Afterwards Tissaphernes pretended to have killed the rebel himself, with the result that Parysatis took cruel vengeance upon the slayer of her favourite son. The Persian troops, instead of attacking the Greeks via a driect assault, decoyed them into the interior, beyond the Tigris, and then attack through trickery. It was a solid and clever plan but, after their commanders had been taken prisoners, the Greeks managed to force their way to the Black Sea. This achievement demonstrated the potential superiority of Greek soldiers against their Persian adversaries. It is thought that this was the reason why Philip II of Macedon formulated his strategy of defeating the Persian Empire by means of a compact and well-trained army: a feat accomplished by his son, Alexander the Great.
The history of Cyrus and of the retreat of the Greeks is told by Xenophon in his Anabasis (where he tries to veil the actual participation of the Spartans). Another account, probably from Sophaenetus of Stymphalus, was used by Ephorus, and is preserved in Diodor. xiv. 19 if. Further information is contained in the excerpts from Ctesias by Photius; cf. also Plutarch’s life of Artaxerxes.