The Great Leader, King Xerxes seceeded the throne with the death of Cyrus the Great.
Xerxes sought to conquer the Greeks at the urging of his aids. At least in part, the reasoning was to make the Empire more secure. Also, he remembered still the Persian defeat at Marathon.
Other reasons given for his want to invade the Greek mainland was supposedly suggested by Mardonius, one of his advisors. He said that Greece was a garden center, which would appeal to the senses of Xerxes.