In 480 BC, the unstoppable expansion of the Achaemenid Persian Empire clashed with an immovable coalition of Greek city-states at the narrow, 100-meter-wide pass of Thermopylae, also known as the Hot Gates. King Xerxes I mobilized a massive force of 120,000 to 300,000 soldiers to punish Athens for backing the Ionian Revolt and absorb Greece into his empire, fully expecting the defenders to flee upon seeing his vast numbers. Instead, King Leonidas I of Sparta anchored a vanguard of 7,000 Greek troops—including his elite 300 citizens, 900 enslaved Helots, and various regional allies—at the bottleneck. Utilizing the heavy phalanx formation, with overlapping bronze-clad wooden shields (aspis) and long ash wood spears, the Greeks easily routed waves of lightly armored Persian infantry and even the elite 10,000 "Immortals," defying Xerxes' dem ...