Leonidas was a 5th century B.C. Spartan military king who bravely led a small force of Greeks -- mostly Spartan (the famous 300),
but also Thespians and Thebans -- against the much larger Persian army of Xerxes, at the pass of Thermopylae, in 480 B.C.
during the Persian Wars. In his book on the Persian Wars, Greco-Persian Wars [it's the same war, but with a less biased name, since 'Persian', as opposed to 'Greco-Persian' marks the Persians as the enemy], Peter Green suggests only the Thebans and Thespians accompanied Leonidas into battle because they were the only other Greeks who volunteered.
According to Herodotus, Leonidas had been warned by the Delphic oracle that either Sparta would be destroyed or their king would lose his life. Leonidas chose the second alternative.
All the Spartans and Thespians died, including Leonidas, whose corpse the Persians mangled. Herodotus says the Thebans, who had never wanted to be there, surrendered when Leonidas was killed.
Leonidas was the half-brother of the late King Cleomenes I of Sparta. After Cleomenes' probable suicide, Leonidas was made king because Cleomenes had died without a son or another, closer male relative to reign as his successor. There was another tie: Leonidas was also married to Cleomenes' only child and heir, the wise Gorgo. Herodotus says this was part of the reason he became king. Sparta had two hereditary kings at a time. One group was the Agiad and the other, the Eurypontid. As kings from the Agiad line, both Cleomenes and Leonidas claimed Heracles (Hercules) as ancestor.