 However, he died before the invasion could begin.The throne of Persia passed to his son Xerxes I, who quickly restarted the preparations for the invasion of Greece, including building two pontoon bridges across the Hellespont.In 481 BC, Xerxes sent ambassadors around Greece asking for earth and water as a gesture of their submission, but making the very deliberate omission of Athens and Sparta (both of whom were at open war with Persia).