What happened at the Battle of Thermopylae?

In 480 BCE, the Persian king Xerxes I led a massive invasion of Greece to expand his empire. At the narrow mountain pass of Thermopylae, a small Greek force led by the Spartan king Leonidas attempted to halt the Persian advance.12

For two days, the Greeks successfully defended the pass, using the narrow terrain to neutralize the Persians' superior numbers and inflicting heavy casualties. However, the tide turned when a Greek citizen named Ephialtes betrayed his countrymen by informing Xerxes of a mountain path that allowed the Persians to outflank the Greek position.123

Upon learning of the encirclement, Leonidas ordered the majority of the Greek army to retreat. He remained behind with a small contingent—including his 300 Spartan bodyguards—to resist the Persians, ultimately fighting to the death. This victory allowed Xerxes to continue his march into southern Greece. Today, the battle is remembered as a profound example of courage and persistence against overwhelming odds.132