Leonidas I, whose name means "son of the lion" in Greek, was the legendary king of the Spartans who heroically fought and died defending Greece from a Persian invasion at the Battle of Thermopylae in August of 480 BC.
The 17th of the Agiad line of Spartan kings, Leonidas was one of the sons of King Anaxandridas II, and according to legend, was a direct descendant of Herakles. According to Herodotus, Leonidas's mother failed to produce offspring for so many years that the ephors (the five annually elected administrators of Sparta) begged Leonidas's father Anaxandrias II to cast aside his wife and take another. However, Anaxandrias, feeling his wife was blameless, refused, whereupon the ephors prevailed upon him to take a second wife without divorcing his first wife.
The new wife immediately bore a son, Cleomenes, but thereafter the first wife bore several sons of which Leonidas was the second. Because Leonidas was the third in line to the throne and not expected to become king, he was not exempted from attending the agōgē, and thus was one of the few Spartan kings to have been subjected to full brutality of the harsh training regimen for ordinary Spartan boys.
But Leonidas's elder brother Dorieus went into exile out of anger that Cleomenes was chosen as king when Anaxandrias died in 520 BC, and then Cleomenes was deposed by reason of insanity, causing Leonidas to become king of Sparta around the age of 40 in 489 BC or so. Sometime before becoming King, he had married Cleomenes' daughter, Gorgo, a woman at least 20 years his junior, who would become his queen.
In 481 BC, Leonidas was chosen to lead the combined Greek forces against the Persian invasion. We know that Leonidas must have built up a formidable reputation in battle by this time, since he was personally requested by the other Greek city states to be their leader. According to Herodotus, the Spartans consulted the Oracle at Delphi, which warned them, in perfect hexameter verse:
O ye men who dwell in the streets of broad Lacedaemon!
Either your glorious town shall be sacked by the children of Perseus,
Or, in exchange, must all through the whole Laconian country
Mourn for the loss of a king, descendant of great Herakles.
In August, 480 BC, Leonidas led a force of 300 Spartiates and 900 Helots to join with several thousand hoplite soldiers from the other Greek city states and meet the Persians at Thermopylae. On the eve of the battle the Persians sent an emissary to parlay with Leonidas, promising generous treatment to the Greeks if only they would surrender their weapons, to which Leonidas famously replied, with classic Laconic brevity, Molon labe! (μολὼν λαβέ - "Come and get them!").
Under Leonidas's leadership the Greeks repulsed the Persian assault for two days. But on the third day, a Malian named Ephialtes betrayed his fellow Greeks by showing the Persians a secret path through the mountains to the rear of the Greek position, whereupon Leonidas ordered the rest of the Greeks to withdraw, leaving only the Spartans, the Helots, and 700 Thespians who refused to leave.
Surrounded and vastly outnumbered, the Greeks fought to the last man, and Leonidas died in the battle. According to Herodotus, the Persian king Xerxes ordered his remains desecrated by having his head cut off and put on a stake and his body crucified.
Leonidas almost immediately became the subject of worship as the center of a Greek hero cult that lasted at least until the 1st century AD, and remains a national hero in Greece to this day. In 1955, King Paul of Greece erected a statue of Leonidas at Thermopylae bearing the inscription ΜΟΛΩΝ ΛΑΒΕ ("Come and get them!"). Another statue, also with the inscription ΜΟΛΩΝ ΛΑΒΕ, was erected in Sparta itself (the modern-day town of Sparti) in 1968.
"Go, tell the Spartans, stranger passing by,
That here, obedient to their laws, we lie.
–– epigram by Simonides on the epitaph at Thermopylae