Throughout ancient Greece, the city-state of Sparta was known for producing the most fearsome infantry that the Greek world had ever seen. There are legends that say the Spartan infantry were matched only in sheer combat prowess and coordination by the Roman legions, though historically, the legions only attained their famed discipline and combat coordination after the successful invasion of Sparta.
The legend of Spartan power and invincibility only became widespread across the world after the epic event called the Battle of Thermopylae, a battle that was a long-term strategic victory for Sparta even if it was an immediate tactical defeat for the 300 Spartans that engaged the invaders from Persia. The 300 soldiers walked into battle not knowing that their ferocity and gallantry would seal the reputation and legacy of the word 'Spartan' for all eternity. At Thermopylae...at the so-called “gates of fire” --- a myth was born and a legend was sealed.
Of course, this would not have happened had it not been for events prior to the battle itself. Darius, ruler of the Persian Empire, had attempted to conquer Greece as punishment for Athenian citizens who aided in the rebellion of a minor Greek town within Darius' domain. The Battle of Marathon was the result of that invasion, with Persian forces pushed back by Athenian infantry. At a later date, the Persian king Xerxes, ruler of the greatest empire of his time, schemed to make it greater still, by conquering the city-states of Greece. However, modern historians who refute this claim say that Xerxes was a known patron of culture and the arts, with hardly any military campaigns under his belt prior to the invasion. Some experts in Persian and Greek history have attributed the alleged decision of Xerxes to invade Greece as a sign of status anxiety.
Xerxes felt the pressure that was placed on his shoulders after the Athenians routed his father's army at Marathon. Following their defeat, it became clear that the Persian Empire was not superior in terms of military capability. This resulted in status anxiety, as the Persians were not known for their failures, as evidenced by their expansive empire. In theory, Xerxes may simply have grown tired of his courtiers and the Persian aristocracy that continually hounded him to invade Greece, supposedly to avenge his father's defeat. However, the other half of this equation, the Spartans, also experienced tremendous status anxiety.
Leonidas had tried to send men to Marathon, in response to Athens' request for help, but there were complications. The Spartans could not go to war during a religious celebration, which resulted in Spartan warriors arriving at Marathon too late. This resulted in greater glory for Athens, Sparta's rival city-state. This, Leonidas understood, threatened the superiority of the Spartan people and put the whole of the population under a mild climate of status anxiety right after Athens was acclaimed as the “savior of all Greece.” So when the Persian invasion came, Leonidas wanted to be there and meet the invaders with his spear. Again, duties to the gods of Greece and the need for Spartan warriors to remain and keep their slave population under control got in the way. The Greek politicians wanted engage in negotiations with the Persians because of their fear and anxiety. Leonidas chose to requisition a small force that will join him in the battle against the invaders.
The arrival of the Spartans was, undoubtedly, a morale boost for the allied Greek armies. The mere mention of a Spartan fighting a battle was enough to instill fear and anxiety in any opposing army, with that ferocious reputation made more magnified by the blood red cloaks and perfect phalanx of the Spartan warrior elite. On the first day of the Battle of Thermopylae, the Spartans taught the Persian hordes a lesson the other Greeks knew all too well. Spartans were not mere men. Spartans were a well-oiled killing machine that was fully ready to engage in mass slaughter. As one of Xerxes' aides said to the Persian king, “... to subdue the Spartans is to ensure no nation in Greece would dare challenge you.” Of course, as the Persians quickly found out, subduing the proud Spartans was not that simple.
While the Spartans were not the only ones to have fought on the front lines during the battle, they were the ones that took the brunt of the incoming Persian force. However, records written by the Persians themselves described the horror, fear, and anxiety that the prospect of facing a Spartan caused the invading troops. It is arguable whether or not Xerxes' generals understood the inevitable strategic consequences of taking on the Spartans by sending wave after wave of soldiers against them, but the terrain made it impossible to route the Leonidas' defensive line. Certainly fatigue would have set in for the Greeks eventually, but the problem for the Persians was just how much of their army would die before that happened.
Of course, greed being the motivator that it is, caused a betrayal among the Greeks. A local Greek named Ephialtes had betrayed the location of a small pass that the Persians could use to flank the Spartan line and surround the Greeks. There were defenders at that pass, but they ran at the sight of Persian troops. The records are being disputed about whether Leonidas ordered the other Greeks to retreat as a tactical decision, or if the Spartans were abandoned to die. Regardless, the surviving Spartan force, along with a Thespian force that refused to abandon them to die alone, remained.
On the third day of battle, with Persian morale low because of the fear and anxiety that the Spartan slaughter of their comrades caused, the final push was planned. The Spartan and Thespian forces, routed and surrounded, were inevitably all killed, though such was the fear and anxiety they caused that Xerxes could not make his troops commit to one last engagement, even as the Spartans were reduced to fighting with their hands and teeth. Instead, he ordered them to be shot down with arrows.
However, while it is considered a tragic defeat, the Battle of Thermopylae was still regarded as a strategic success. The death of one of their kings (Sparta was a diarchy, with two kings at any given time) motivated Sparta to raise and deploy more troops into battle. Even by the time the Battle of Plataea was fought, the Persian commanders and troops were still feeling the fear and anxiety they felt at Thermopylae, affecting their ability to fight a Spartan force of 5,000, along with several thousand more Greeks from Athens, Thespiae, and other city-states. The historical record is unclear on this point, but it is highly probably that Spartans rallied and inspired their troops by invoking the memory of Leonidas and his 300.