It was a Persian custom that the king, before he went out of his dominions on an expedition, should nominate a successor.

Darius must have done this before his campaign in Thrace and Scythia; and if Xerxes was then, as is probable, a mere boy, it is impossible that he should have received the appointment.

Artobazanes, the eldest of all Darius's sons, whose mother, a daughter of Gobryas, was married to Darius before he became king, was most likely then nominated, and was thenceforth regarded as the heir-apparent.

When, however, towards the close of his reign Darius again proposed to head a foreign expedition, an opportunity occurred of disturbing this arrangement, of which Atossa, Darius's favorite wife, whose influence over her husband was unbounded, determined to take advantage.

According to the law, a fresh signification of the sovereign's will was now requisite; and Atossa persuaded Darius to make it in favor of Xerxes.
The pleas put forward were, first, that he was the eldest son of the king, and secondly, that he was descended from Cyrus.