Battle at the Hot Gates was about to begin. Xerxes, parenthetically, had already disregarded one formality.
Before an invasion, Persia's Great King would typically send an emissary to cities and towns in the path of destruction. Their purpose was to demand "earth and water." If the about-to-be-crushed agreed to give their land - and their water - to the king, they could avoid the battle but would live as subjects. Any thought of freedom, however fleeting, would no longer apply.
Xerxes' father had sent such emissaries to Greek city-states before the battle of Marathon. Some of the Greeks capitulated. Athens and Sparta responded very differently:
[The Persian emissaries] were thrown, at Athens, into the pit of punishment, at Sparta into a well, and bidden to take therefrom earth and water for themselves, and carry it to their king. (Herodotus, Histories, 7.133)
Anticipating a similar response to this invasion, Xerxes did not bother to send emissaries.
For the first four days, at Thermopylae, Xerxes waited. He thought the Greeks would run, not fight. No longer able to waste supplies, the king - seething with rage, according to Herodotus, that the Greeks were actually going to resist him - allegedly sent a directive:
Hand over your arms!
Leonidas, unafraid and unimpressed by the order, is said to have sent back a two-word (in Spartan Greek) response: Molon Labe (mo-lone lah-veh)! Translated into English:
Come and get them!
Xerxes sent in the Medes on the first day of battle, to do just that. Correlating existing evidence, scholars believe this was either the 17th of August or the 17th of September, 480.
Ordering his troops to take the Spartans alive, Xerxes expected a rout. But Sparta's defenders, pretending to retreat or flee, had a different plan. Once the Medes broke rank, and started to run after the Spartans, the defenders turned toward their enemies and got on with their jobs, savagely fighting and killing their opponents. Spartans had employed such tactics before, and their longer spears were exceedingly effective.
Surprised at the defenders' tenacity, Xerxes sent his Immortals to the battlefield. They endured similar tactics and met the same fate.
Resting overnight, soldiers recommenced fighting the next morning. Still the Persian coalition made no progress. Then a traitor, named Ephialtes, had a conversation with Xerxes.
Momentum was about to turn - irreversibly - against the Greeks.