Why was the battle of Thermopylae considered to be a turning point of the Persian War?

i hope this helps?

The Battle of Thermopylae took place at Thermopylae. Greeks were able to hold off Persian invaders for 3 days until a Greek traitor showed the Persians a way over the mountains. ... Xerxes' defeat was a turning point in history because it ensured that Greek culture could continue to thrive in the west.