Cyrus the Younger
After their victory in the Peloponnesian War (431–404 BC), the Spartans became the dominant power in the Aegean, creating a new threat for the Achaemenid Empire. The Spartans then antagonized the Achaemenid king Artaxerxes II by militarily supporting the rival bid of his brother Cyrus the Younger, their ally during the Peloponnesian war, leading to the campaign of the Ten Thousand deep into Achaemenid territory in 401-399 BC. Cyrus the Younger failed, but the relationship between Sparta and the Achaemenid Empire remained adversarial.