The Battle of Thermopylae was caused by the invasion of Greece by a Persian army under Xerxes. In an effort to block the Persian army from advancing into southern Greece, the vastly outnumbered Greeks took a stand at a strategic pass, inflicting many casualties before they were betrayed and overrun.
After his father Darius I lost the Battle of Marathon to the Greeks in 490 B.C., Xerxes mounted a much larger invasion, intending to overwhelm the Greek mainland. With an accompanying offshore fleet and an army of over 100,000 soldiers, Xerxes headed down the east coast of Greece towards Athens, Sparta and other Greek city-states in 480 B.C. King Leonidas of Sparta, with 300 Spartans and about 7,000 other Greek allies, headed north to meet the army of Xerxes. They decided on the pass of Thermopylae to make their stand because it was a narrow, easily defensible passage bounded by a cliff and the sea. For two days, the Greeks held off the Persians. At the end of the second day, however, they were betrayed by a local named Ephialtes, who told the Persians about a flanking trail over the mountains.
King Leonidas ordered most of the Greek army to retreat. To protect them, he remained at Thermopylae with his 300 Spartans and about 1,000 other Greek allies. The Persian army slaughtered most of the defenders, including Leonidas and all the Spartans. Historians estimate battle casualties of about 20,000 Persians and 2,000 Greeks.