Leonidas I in Wikipedia
Leonidas (pronounced /liːˈɒnɨdəs/, Greek: Λεωνίδας, Leōnidas, literally "lion's son") was a king of Sparta, the 17th of the Agiad line, one of the sons of King Anaxandridas II of Sparta, who was believed in mythology to be a descendant of Heracles, possessing much of the latter's strength and bravery. The date of Leonidas' birth is not known, although Paul Cartledge argues he must have been born around 540 BC. This, however, would have made Leonidas 60 at his death and his wife would have been more than 40 years younger than he - something that would have been extremely exceptional in Spartan society. Furthermore, the fact that his son was still a minor at the time of his death is another indicator that he may have been considerably younger than Carledge hypothesizes since Spartan men were expected to marry before the age of 30. Had Leonidas been born in 540, he should have had adult sons and even grandsons by 480.
Significantly, Leonidas was not his father's eldest son. According to Herodotus Leonidas' mother was his father's niece and had been barren for so long that the ephors, the five annually elected administrators of the Spartan constitution, tried to prevail upon King Anaxandridas to set aside his wife and take another. Anaxandridas refused, claiming his wife was "blameless," whereupon the ephors agreed to allow him to take a second wife without setting aside his first. This second wife, a descendent of Chilon the Wise, promptly bore a son, who was named Cleomenes. However, one year after Cleomenes' birth, Anaxandridas' first wife also gave birth to a son, Dorieus.  Leonidas was the third son of Anaxandridas' wife, and either the elder brother or twin of Cleombrotus.  Because Leonidas was not heir to the throne, he was not exempt from attending the "agoge," the public school that the sons of all Spartans had to complete in order to qualify for citizenship.  Leonidas was thus one of the few Spartan kings to have ever undergone the notoriously harsh training of Spartan youth.
Cleomenes I succeeded to his father's throne somewhere between 520 and 516 BC.  Dorieus was so outraged that "the Spartans" had preferred his half-brother over himself that he found it impossible to remain in Sparta. He made one unsuccessful attempt to set up a colony in Africa and, when this failed, sought his fortune in Sicily, where after initial successes he was killed.  Leonidas' relationships with his bitterly antagonistic elder brothers is unknown, but he married to Cleomenes' daughter, Gorgo sometime before coming to the throne in 490 BC. 
While nothing more concrete is known about Leonidas' life before he came to the throne or even of his reign, it is important to keep the following in mind. Leonidas was clearly heir to the Agiad throne and a full citizen at the time of the Battle of Sepeia against Argos at Sepeia (c. 494 BC). Likewise, he was a full citizen when the Persians sought submission from Sparta and met with vehement rejection in or around 492/491 BC. His brother had already gone mad and fled into exile when Athens sought assistance against the Persian invasion that ended at Marathon (490 BC). It is reasonable to assume that Leonidas played an active role in all these events given his prominence, particularly after his brother's insanity had become manifest in treasonable activities. Furthermore, Plutarch has recorded the following: "When someone said to him: 'Except for being king you are not at all superior to us,' Leonidas son of Anaxandridas and brother of Cleomenes replied: 'But were I not better than you, I should not be king.'"  As the product of the agoge, Leonidas is unlikely to have been referring to his royal blood alone but rather suggesting that he had - like his brother Dorieus - proven superior capability in the competitive environment of Spartan training and society and that he believed this made him qualified to rule.
Certainly it is historical fact that Leonidas was elected to lead the combined Greek forces determined to resist the Persian invasion in 481. This was not simply a tribute to Sparta's military prowess. Sparta had two kings. If all that was needed was a Spartan commander, Leotychidas of the Eurypontid house would have done just as well. The fact that the coalition wanted Leonidas personally is underlined by the fact that just two years after his death, the coalition preferred Athenian leadership to the leadership of either Leotychidas or Leonidas' successor (as regent for his still under-aged son) Pausanias. The rejection of Leotychidas and Pausanias was not a reflection on Spartan arms. Sparta's military reputation had never stood in higher regard. Nor was Sparta less powerful in 478 than it had been in 481. Furthermore, Pausanias himself had proved an effective commander in his stunning victory over the Persians at Plataea in 479. The rejection of Leotychidas and Pausanias in 478 underline the fact that Leonidas was personally viewed by the Greek city-states in the alliance against Persia not only as the most qualified military commmander but as a man they trusted and respected.
This election of Leonidas to lead the defense of Greece against Xerxes invasion had fateful consequences for Leonidas. Leonidas was killed at the Battle of Thermopylae. Yet it was his stance here that transformed him from a mere historical figure into a legend.
Upon receiving a request from the confederated Greek forces to aid in defending Greece against the Persian invasion, Sparta consulted the Oracle at Delphi. The Oracle is said to have made the following prophecy in hexameter verse:
Hear your fate, O dwellers in Sparta of the wide spaces;
Either your famed, great town must be sacked by Perseus' sons,
Or, if that be not, the whole land of Lacedaemon
Shall mourn the death of a king of the house of Heracles,
For not the strength of lions or of bulls shall hold him,
Strength against strength; for he has the power of Zeus,
And will not be checked till one of these two he has consumed.
In August 480 BC, Leonidas set out to meet Xerxes' army at Thermopylae with a small force of 300, where he was joined by forces from other Greek city-states, who put themselves under his command to form an army between 4,000 and 7,000 strong. There are various theories on why Leonidas was accompanied by such a small force of Spartiates. According to Herodotus "Leonidas and his three hundred were sent by Sparta in advance of the main army, in order that the sight of them might encourage the other confederates to fight and prevent them from going over to the enemy, as they were quite capable of doing if they thought that Sparta was hanging back; the intention was, when the Carneia was over (for it was that festival which prevented the Spartans from taking the field in the ordinary way), to leave a garrison in the city and march with all the troops at their disposal."  Many modern commentators are unsatisfied with this explanation and point to the fact that the Olympic Games were in progress or impute internal dissent and intrigue.
Whatever the reason Sparta's own contribution was just 300 Spartiates (accompanied by their attendants and probably perioikoi auxilliaries), the total force assembled for the defense of the pass of Thermopylae came to something between four and seven thousand Greeks. They faced a Persian army of between 80,000 and 290,000 men-at-arms who had invaded from the north of Greece under Xerxes I.
Xerxes waited four days to attack, hoping the Greeks would disperse. Finally, on the fifth day the Persians attacked. Leonidas and his men repulsed the Persians' frontal attacks for the fifth and sixth days, killing roughly 20,000 of the enemy troops and losing about 2,500 of their own. The Persian elite unit known to the Greeks as "the Immortals" was held back, and two of Xerxes' brothers (Abrocomes and Hyperanthes) died in battle. On the seventh day (August 11), a Malian Greek traitor named Ephialtes led the Persian general Hydarnes by a mountain track to the rear of the Greeks. At that point Leonidas sent away all Greek troops and remained in the pass with his 300 Spartans, 900 Helots, and 700 Thespians who refused to leave. Another 400 Thebans were kept with Leonidas as hostages. The Thespians stayed entirely of their own will, declaring that they would not abandon Leonidas and his followers. Their leader was Demophilus, son of Diadromes, and as Herodotus writes: "Hence they lived with the Spartans and died with them."
One theory provided by Herodotus is that Leonidas sent away the remainder of his men because he cared about their safety. The King would have thought it wise to preserve those Greek troops for future battles against the Persians, but he knew that the Spartans could never abandon their post on the battlefield. The soldiers who stayed behind were to protect their escape against the Persian cavalry. Herodotus himself believed that Leonidas gave the order because he perceived the allies to be out of heart and unwilling to encounter the danger to which his own mind was made up. He therefore chose to dismiss all troops except the Thespians and Helots and save the glory for the Spartans.
Of the small Greek force, attacked from both sides, all were killed except for the Thebans, who surrendered. Leonidas was killed, but the Spartans retrieved his body and protected it until their final defeat. Herodotus says that Xerxes' orders were to have Leonidas' head cut off and put on a stake and his body crucified. This was considered sacrilegious.
The tomb of Leonidas lies today in the northern part of the modern town of Sparta. Additionally, there is a modern monument at the site of the Battle of Thermopylae, called the "Leonidas Monument" in his honor. It features a bronze statue of Leonidas. A sign, under the statue, reads simply: "Μολών λαβέ" ("Come and get them!") which the Spartans said when the Persians asked them to put down their weapons. The hero cult of Leonidas survived at Sparta until the age of the Antonines