What was the cause of the Thermopylae battle?

Answer 1

The Battle of Thermopylae happened as a result of the decision of a group of Greek cities, united against the Persian invaders, to block the invading army at the most defensible point. With the terrain at Thermopylae favoring defense, the Greek army, including a contingent of Spartan warriors, assembled there in the hope of stopping the Persians outright, or at least to buy time for further defensive preparations to be made.

Answer 2

Persian king Xerxes had invaded mainland Greece and captured the northern half of it in 480 BCE. In the spring of 479, the southern cities sent out their contingents which united at Plataia to defeat the Persian army and its northern Greek allies.

Answer 3

The Persians wanted to get revenge somewhat because of their lost in the previous Battle of Marathon.

Well, the cause is the Persians thought it was there god-given-right to rule the world. So, Greece became one of their first targets.