The 480 BCE battle in which King Leonidas led a small Spartan force against Persia was the Battle of Thermopylae.
Thermopylae was a narrow pass in central Greece during Xerxes I’s invasion of Greece. Leonidas commanded a coalition force that included 300 Spartan citizens, alongside troops from several other Greek communities. The famous “300” therefore does not represent the entire Greek defense. Thousands of allied soldiers fought during the battle, especially in its opening stages.
The defenders used the terrain to limit the numerical advantage of the Persian army. The position was eventually outflanked after a local man, Ephialtes, revealed a mountain path to the invaders. Leonidas dismissed many allied troops and stayed with a rearguard that was destroyed.
Thermopylae was a Persian victory, but the delay became a lasting symbol of courage and sacrifice. Later Greek victories at Salamis in 480 BCE and Plataea in 479 BCE helped end Xerxes’ invasion.