The Battle of Thermopylae was fought in 480 BCE between the Achaemenid Persian Empire under Xerxes I and an alliance of Greek city-states led by Sparta under Leonidas I. Lasting over the course of three days, it was one of the most prominent battles of both the second Persian invasion of Greece and the wider Greco-Persian Wars.

Around the start of the invasion, a Greek force of approximately 7,000 men led by Leonidas marched north to block the pass of Thermopylae. Ancient authors vastly inflated the size of the Persian army, with estimates in the millions, but modern scholars range it between 120,000 and 300,000 soldiers. They arrived at Thermopylae by late August or early September; the outnumbered Greeks held them off for seven days (including three of direct battle) before their rear-guard was annihilated in one of history's most famous last stands. During two full days of battle, the Greeks blocked the only road by which the massive Persian army could traverse the narrow pass. After the second day, a local resident named Ephialtes revealed to the Persians the existence of a path leading behind the Greek lines. Subsequently, Leonidas, aware that his force was being outflanked by the Persians, dismissed the bulk of the Greek army and remained to guard their retreat with 300 Spartans and 700 Thespians. It has been reported that others also remained, including up to 900 helots and 400 Thebans. With the exception of the Thebans, most of whom reportedly surrendered, the Greeks fought the Persians to the death.