But while Darius was making preparations against Egypt and Athens, a great quarrel arose among his sons concerning the chief power in the land. They held that before his army marched he must declare an heir to the kingship according to Persian law. Three sons had been born to Darius before he became king by his first wife, the daughter of Gobryas, and four more after he became king by Atossa daughter of Cyrus. Artobazanes was the oldest of the earlier sons, Xerxes of the later; and as sons of different mothers they were rivals. Artobazanes pleaded that he was the oldest of all Darius' offspring and that it was everywhere customary that the eldest should rule; Xerxes argued that he was the son of Cyrus' daughter Atossa and that it was Cyrus who had won the Persians their freedom.