There's a very good reason why the battle of Thermopylae has been held in such high esteem by historians...
True, the physical and material impact of Thermopylae on the war was quite small. The losses suffered by the Persian army were considerable but not enough to keep it from sweeping through the rest of Greece and beyond. The stand at Thermopylae was simply a delaying action meant to buy time for the city states beyond the pass so that they could prepare their defenses or in Athens' case, prepare for evacuation. The successful delaying action at Thermopylae also coincided with the successful blockade of the Persian fleet just off the coast, proving that the Hellenic efforts on both land and sea were mutually dependent. Add to the mix a little help from mother nature in the form of a massive storm that claimed a large number of Persian ships and suddenly Xerxes' unstoppable Persian juggernaut found itself stumbling in the face of a relatively small amount of resistance.
On the other hand the psychological significance of Thermopylae was positively enormous. The idea that such a relatively small number of men could hold off a vastly numerically superior enemy for three days seems quite reasonable to us but back then there were few armchair generals around with several thousand years of military history on their bookshelves to provide Leonidas or Xerxes with some historical references. Clearly the qualititative differences between an army of professional and citizen soldiers and that of conscripts and slaves had never really been put to the test prior to Thermopylae (Marathon excepted). Propaganda or historical revisionism aside Xerxes must have been fuming with rage over the inability of his Persian host to break through the pass at Thermopylae and the Athenian navy off the coast. Xerxes lost several relatives and a good portion of his best troops at Thermopylae and his subsequent dessecration of Leonidas' body, a fellow monarch who died honorably in combat, was a shockingly disrespectful act and wholly uncharacteristic of a Persian emperor. Victory or not Xerxes was obviously beside himself over the losses he suffered during the battle.
Looking beyond the act of defiance in the face of overwhelming odds you must also consider that the ancient Greeks took death alot more seriously than those of us living in the monotheistic dominated present where the idea that an immortal soul can be granted eternal peace in a place called Heaven. The ancient Greeks believed the after-life was a dark and dreadful place where the souls of both the decent and the wicked wandered aimlessly in Hades. The idea that the Greeks who fought at Thermopylae to the bitter end would willingly sacrifice their bodies and send their souls to such a place makes their sacrifice even more impressive.
Above all the notion that a Spartan king, a few hundred hand picked Spartans, several hundred Thespians and other Greeks, all free men who in the past hated each other almost as much as they hated foreigners, would choose to lay down their lives in a seemingly futile act of defiance is simply astonishing. From the Persian perspective this collective act of self-sacrifice must have been mind boggling. Who knows what the average Persian soldier thought of the Greeks after that battle? Prior to Thermopylae one could chalk up Darius' loss at Marathon to bad luck or chance but three days at Thermopylae were awfully hard to dismiss. Persian morale must have taken a serious dive after Thermopylae, just imagine the rumors and expressions of wild imagination that spread amongst Xerxes' decidedly superstitious and uneducated army after the battle.
Similarly, we can look to the Japanese bushido code and the advent of the kamikaze pilot during WW2 as being equally dubious in terms of their military value but their combined impact upon Allied morale cannot be stressed enough. In light of the incredible tenacity with which the Japanese fought the US on Iwo Jima and Okinawa the US government was desperately looking to end the conflict prior to an actual invasion of the Japanese home islands.
Never underestimate the value of morale in warfare.
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