Leonidas ordered most of the Greek army to retreat from Thermopylae while he and a small rearguard stayed behind to buy time and prevent a disastrous pursuit.

226:36they saw that they would be surrounded, and  King Leonidas knew that the battle was lost. He ordered the rest of the army to retreat,  and stayed behind with his royal bodyguard and some assorted Thespian and Theban allies  to hold the pass a little while longer. This was an act of personal bravery  by the Spartans who remained, but it was also a tactical necessity.  If the whole army retreated,

227:13Persian heavy cavalry would have pursued  them and likely cut them down to a man. Spartan hoplites had also been raised within  a brutal warrior culture that demanded their sacrifice in battle. Men who returned from  losing battles were often treated as cowards and shunned in Spartan society. Even two men  who Leonidas sent away from Thermopylae due to injuries were treated as outcasts when they  returned to the city, as Herodotus describes.

227:54When Aristodemus returned to Sparta, reproach and  disgrace awaited him. No Spartan would give him a light to kindle his fire, or so much as address  a word to him. All spoke of him as the coward. Another of the 300 is likewise said to have  survived the battle, a man named Pantites. He, they said, on his return to Sparta found himself  in such disesteem that he hanged himself.

228:31For Xerxes, the battle at Thermopylae had been  a resounding victory. He had surprised his enemy before their reinforcements were able to  arrive, outmaneuvered them on their home soil, and forced them to retreat from the choke point  they had hoped to defend. And, thrown into the bargain, he had cut off and killed one of the  two kings of Sparta, meaning that a child now sat on one of its thrones. For the Greeks,  Thermopylae was a sorry defeat tactically and