In the Battle of Thermopylae of 480 BC, an alliance of Greek city-states fought the invading Persian army at the pass of Thermopylae in centralGreece. Vastly outnumbered, they held back the invader in one of history's most famous last stands. A small force led by King Leonidas ofSparta blocked the only road through which the massive army of Xerxes I could pass. After three days of battle, a local resident named Ephialtes betrayed them by revealing a mountain path that led behind their lines. Dismissing the rest of the army, King Leonidas stayed behind with 300 Spartans and 700 Thespian volunteers. Though they knew it meant their own deaths, they held their position and secured the retreat of the other forces. The Persians succeeded in taking the pass but sustained heavy losses, extremely disproportionate to those of the Greeks. The fierce resistance of the Spartan-led army offeredAthens the invaluable time to prepare for a decisive naval battle.