"Such was the successless armament of Xerxes, described by Herodotus"

Xerxes I
Public DomainXerxes I - Credit: Guillaume Rouille (1518?-1589)
Xerxes I of Persia, also known as Xerxes the Great (519 BC-465 BC), was the fourth king of the Achaemenid Empire. In 480 BC he led an attack on the Greek mainland.  His army has been estimated at about a million men, with 10,000 elite warriors named the Persian Immortals.  He captured Sparta and Athens.  It is not clear whether he purposefully burned Athens to the ground, or whether the fire was started by accident, or even purposefully, by the fleeing citizens. 

Despite these military successes, trouble was brewing at home, and Xerxes was forced to return with his army to prevent a revolt in Babylon.  The army he left behind in Greece was attacked and defeated, and the Persians were forced to retreat.