In 480 BC, a Greek army led by 300 Spartans held off a Persian army numbering in the hundreds of thousands at Thermopylae, a chokepoint along the Greek coast, for seven days. On the seventh day, the Persians found a route through the nearby mountains, allowing a portion of the Persian army to flank the Greeks. The Spartan king leading the Greek army dismissed the bulk of his army on a retreat, then stayed to fight to the death against the Persians.
I likely don't need to tell you the full story of the Spartans at Thermopylae because it has become legend.