After the Persians failed to conquer the Greek mainland in 479 B.C.E., the Persian Empire and its Achaemenid leaders began a slow decline. The kings who followed Xerxes were weak rulers.
At the same time, the Greek city-states grew stronger after Philip of Macedonia organized them into the Hellenic League. He named himself and his descendants leaders of the league. Philip was murdered in 336 B.C.E. Two years later, his son, Alexander, later called Alexander the Great, decided to invade Persia.
