What empire did the Greeks fight against at Thermopylae?

What empire did the Greeks fight against at Thermopylae? They fought against the Persian Empire. More precisely, the opposing power was the Achaemenid Empire, the Persian dynasty that ruled a vast territory stretching from parts of southeastern Europe and Egypt across western and central Asia.

The Battle of Thermopylae took place in 480 BC during Xerxes I’s second invasion of Greece. The invasion followed the earlier Persian campaign that ended with the Greek victory at Marathon in 490 BC. Xerxes sought to punish Greek cities that had supported the Ionian Revolt and to expand Persian control over mainland Greece.

The Greek defenders were not a single unified nation in the modern sense. They came from a temporary alliance of independent city-states, including Sparta, Thespiae, Thebes, and others. Their defense at Thermopylae was coordinated with a naval stand at Artemisium, because controlling both the pass and the nearby sea route was essential to slowing the Persian advance.

Thermopylae ended in a Persian victory, but the invasion did not succeed permanently. Greek forces later defeated the Persian fleet at Salamis and the Persian army at Plataea in 479 BC.