It was Themistokles, the most fightproofen warlord of Athen, who took the Thermopyls - a small pathway between hills on the one and the sea on the other side - as the best place for the first clash against the incoming persians. At August 480 BC Leonidas, one of the two kings of sparta, behest to build an outpost there. The Phocis build a wall at the smallest point of the pathway, Leonidas places most of his men behind it. Also 1000 Phocis watched s small path wich led behind the the wall of the greeks.
Xerxes waited with his strike 4 days long, while he tried to wheedle the spartans to give up. Then he send his men against the the pathway. The spartans took position in form of a phalanx in front of the wall and fought off one persian wave after the other - even Xerxes' elite troops the "Undying".
At the evening of the second day in the clash the betrayer Ephialtes of Trachis showed Xerxes the small path which le behind the greek wall and was watched by the phocis. The persian king moved a great piece of his army to the path as fast as he could, to bypass the strong spartian phalanx. The phocis couldnt stand their ground and the persians quickly overrun the path. Most of the greek wanted to withdraw, but the spartians didnt know the word. So they held brave with the Thespiae's while the rest of the greeks fleed from the clashfield.
This was not only brave, but also a wise tactical choice : Had the spartans fleed too, the persians ridery would have slaughtert the organized greeks. Xerxes behest his man to break through the spartans. Now the spartans came to the widest point of the pathway, to maximise the losses of their foe. In the following fight Leonidas was killed. The last living fighters got behind the wall to make a last stand. The thebanians gave up to the persians, while the spartans and thespia's fought to the last man.
The clash at Thermopyale ended in a lose for the greeks. But through theof the spartas they gained worthy time to regroup their troops and mobilise more fighters.