Herodotus, 7.175-239: The Battle at Thermopylae

First, a word about the badasses from Sparta and Athens.  The Spartans had killed some envoys from Xerxes who were offering a peaceful invasion.  Sparta then sent two messengers to Xerxes, allowing them to be killed in response.  But brought before the king, the messengers refused to bow to him (7.135-136):

[Hydarnes] entertained them by serving them a feast during which he asked them, "Lacedaemonians, why are you trying to avoid becoming the King's friends?  You can see that the King knows how to honor good men when you look at me and the state of my affairs.  This could be the same for you if only you would surrender yourselves to the King, since he would surely think you to be good men and allow each of you Greek territory to rule over."  To this they replied, "Hydarnes, you offer us this advice only because you do not have a fair and proper perspective.  For you counsel us based on your experience of only one way of life, but you have had no experienced of the other: you know well how to be a slave but have not yet experienced freedom, nor have you felt whether it is sweet or not.  But if you could try freedom, you would advise us to fight for it, and not only with spears, but with axes!"

After giving that answer to Hydarnes, they traveled inland to Susa and gained an audience with the King.  At first the King's bodyguards ordered them and actually tried to force them to prostrate themselves before the King; but they refused to do so, saying that they would never do that, even if the bodyguards should try to push them down to the ground headfirst, since it was not their custom to prostrate themselves before any human being, and besides, that was not the reason for which they had come.

When the Hellenes were looking for allies to fight against the invasion, they came to Syracuse for help.  Gelon of Syracuse was willing to help, but only if given command either of the land army or the navy.  The Athenian delegation (7.161):

And now the envoy sent by the Atheneians interjected before the one from the Lacedaemonians could reply: "King of the Syracusans, Helalls did not send us to you because it needed a leader, but rather because it required an army.  But you have given us no indication that you would send an army unless you were to become the leader of Hellas; to command its forces is what strongly attracts you.  Now as long as you were asking to lead the entire force of the Hellenes, we Athenians were content to keep quiet, knowing that the Laconian would be capable of answering for us both.  But now that your leadership over the whole force has been ruled out and you ask to command the fleet, you should know that this is the situation: even if the Laconian yields the command of hte fleet to you, we shall not yield it to you.  For that command belongs to us, unless the Lacedaemonians want it.  Now if they want to lead the fleet, we shall not oppose them, but we shall allow no one else to command it.  For if we yield our leadership to the Syracusans, it would then be all for nothing that we have acquired the greatest naval force of all the Hellenes-- we, the Athenians, who represent the most ancient nation of the Hellenes and are the only ones who did not migrate.  Even Homer the epic poet said that the best man to go to Troy and to draw up and marshal the troops was one of ours, so we cannot be reproached for asserting this claim."

The Greeks scouted the terrain and settled for the narrow pass by the city of Thermopylae to set up their defenses, as it was the only rational path through the mountain ranges to the southern Greek peninsulas.  They set up to fight despite the warnings of the oracles.  They caught a break when a storm smashed a few hundred of the Persian ships.  On top of that, several Persain ships sailed into the wrong harbor and were taken by the Greeks.

Finally, Xerxes' army approached the pass.  On the other side, the defense forces were led by Leonidas of Sparta (a direct descendant of Herakles), and were comprised of several thousand from the different cities.  The first waves of Persian soldiers were slaughtered, as their great numbers were of no use in the narrow terrain.  "Indeed, the Hellenes made it clear to everyone, and especially to the King himself, that although there were many in his army, there were few real men" (7.210.2).  Finally, they sought a different way through, taking the even narrower Anopaia Path through the mountains.

Leonidas learned of their plans, and sent the majority of his soldiers away to avoid the upcoming slaughter, but kept the 300 Spartans to fight on.  Knowing they were fighting to their death, they fought more valiantly than usual, charging unexpectedly into the oncoming waves of Persians.  Leonidas himself was killed and Xerxes had him beheaded.

In response to the battle, the Persians changed their invasion plans, sending the ships to occupy the Spartans from the south while the army continued to subjugate the Greeks cities on their march from the north.