Why Did the Final Battle at Thermopylae Change Defensive Warfare Forever? The final stand at Thermopylae in 480 BC—where Leonidas and his 300 Spartans, plus allies, held a narrow pass against hundreds of thousands of Persians—proved terrain could neutralize massive numerical superiority, revolutionizing how outnumbered forces fight. By funneling Xerxes' army into a choke point between cliffs and sea, Greeks used the phalanx's shield wall and long spears to slaughter waves of attackers, buying ti