The battle of Thermopylae ( July to September of 480 BC)

why did it happend?
⋅ Xerxes I, commonly known as Xerxes the great and the successor of Darius the great

⋅ Xerxes I, the successor of Darius I, Had invaded Greece in hopes of accomplishing what his predecessor didn't and it was to succesfully invade Greece. he sent heralds to Greek cities, but he skipped over Athens and Sparta because of their previous responses.
Who was the cause?
⋅ Leonidas, The king of Sparta

( statue of king Leonidas at the Archeological museum of Sparta )