How often does a military defeat set a standard for excellence? How often does a defeat create an aura of invincibility? Even a defeat like The Alamo just became a rallying cry, and the Roman republican defeats showed both their weaknesses and their resiliency, not their invincibility.
The only defeat with this result was the Battle of Thermopylae where 300 brave Spartans (and several thousand other Greeks) held off the main Persian army for days while the Greek force collected and came out for the main battle, or while Athens evacuated. What a marvelously successful propaganda effort this was. The Spartan force was slaughtered, but for two men, and other losses were heavy. High command was splintered and thus no expeditionary force was forthcoming. Likewise, if the Greeks had truly meant to hold the pass as a delaying action, why was this force sent instead of the massive force sent to Tempe the year before?
An answer may be that this was a rogue action taken by Leonidas in order to drag the Greeks out of complacency and force an engagement. If a Spartan king and hundreds of his peers met with Persia, surely the main Greek army would come out and fight with them, but of course that did not happen–and yes, the army had been basically assembled, albeit without most Athenian aid since they were manning ships at the time. Sparta lost 298 full citizens, including a king (for comparison, Sparta went to the negotiating table for peace in the Peloponnesian War when just over a hundred Spartans were captured), and with a successful propoganda campaign this catastrophe became a heroic sacrifice for the liberty of Greece. Spartans at the forefront of the campaign, suffering losses, but fighting like madmen and never surrendering, never retreating and only losing when impossibly outnumbered. Furthermore the number 300 stands out because this same propoganda campaign that pushed the Spartans as the saviors of Greece likewise de-emphasized any other states involved. The very fact that it was just 300 redoubled the heroism of this ‘sacrifice’.
Granted, the images of Thermopylae, from Frank Miller’s 300 to Stephen Pressfield’s The Gates of Fire to Herodotus to Plutarch’s Sayings of Kings and Commanders, are visually stunning. When the Persian emissary asks Leonidas for his weapons, the answer is blunt: molon labe . Warned that the Persian archers would cloak the sun with their arrows, Leonidas or another veteran Spartan responded that this is good, they will have their battle in the shade. The original thin red line holding back an onslaught (with other Greeks by their side), and when the Persians found an alternate path, most of the army was sent home while the Spartans and a few others held a hilltop until they were all killed. But looking beyond these images, the battle was a waste. It was a fleeting pause in the Persian advance and nothing more. No decisive land battle happened that year and the deaths were wasted.