 His empire was unstable, with large portions governed by jealous and unreliable satraps and inhabited by disaffected and rebellious subjects.In 334 BC, Alexander the Great began his invasion of the Persian Empire and subsequently defeated the Persians in a number of battles before looting and destroying their capital, Persepolis, by fire in 330 BC.With the Persian Empire now effectively under Alexander's control, Alexander then decided to pursue Darius.