Why did Cyrus the Younger rebel against Artaxerxes II?

Cyrus the Younger’s rebellion against his brother, Artaxerxes II, was primarily driven by his ambition to seize the Achaemenian throne.12

Cyrus was the favorite son of Parysatis, who had long hoped to secure the succession for him rather than for his elder brother, Arsaces (who became Artaxerxes II). Following the death of their father, Darius II, in 404 BCE, Cyrus was accused by Tissaphernes of plotting to murder the new king. Although he was pardoned through his mother's intercession and returned to his satrapy, Cyrus began actively preparing to challenge his brother’s rule.1

To facilitate his coup, Cyrus used a territorial dispute with Tissaphernes regarding Ionian cities as a pretext to assemble a large military force, which included a significant contingent of Greek mercenaries. He also disguised his true intentions by claiming he was preparing an expedition to Pisidia. He eventually revealed his objective to march against Artaxerxes upon reaching the Euphrates River, leading to the Battle of Cunaxa in 401 BCE, where he was killed.12