What is known
Leonidas was an Agiad king of Sparta who commanded the allied Greek force that held the pass at Thermopylae in 480 BCE and was killed there. That sentence contains most of what can be stated without qualification.
He came to the throne some years before the invasion, following the death of his half-brother Cleomenes I. Herodotus, our only substantial source, gives him a genealogy running back to Heracles — a Spartan royal convention rather than a historical record — and reports that he had not expected to rule.
His command at Thermopylae was of an allied force of roughly 5,200 men drawn from across the Peloponnese and central Greece, of whom 300 were Spartiate citizens. The reduction of that force to "the 300" in popular memory is a distortion the numbers in Herodotus do not support.
Thermopylae
Herodotus reports that Leonidas took only men who had living sons, which if true indicates that the assignment was understood to be dangerous from the outset. He also reports an oracle that Sparta must either lose a king or see the city destroyed, and presents Leonidas as acting with that in mind.
Whether the stand was a planned sacrifice or a rearguard covering a withdrawal cannot be determined. The two readings are both defensible from the same text, and this platform does not choose between them. See Thermopylae.
His body was recovered by the Persians and, Herodotus says, treated with a dishonour he regards as out of character for Xerxes. The bones were later brought back to Sparta.
What the sources are
Almost everything comes from Herodotus, writing a generation and a half later, drawing on Spartan and other informants with reasons of their own. Sparta produced no historical literature. Later accounts — Diodorus, Plutarch — are further away and preserve traditions that cannot be independently checked, including most of the famous sayings attributed to him.
The laconic replies for which he is remembered belong to a collected tradition of Spartan sayings assembled centuries afterwards. They are evidence for how Greeks and Romans wanted Spartans to have spoken.
Reception
No ancient commander has been recruited more often or by more incompatible causes. Leonidas has been made a symbol by nineteenth-century nationalisms, by both sides in the Second World War, and by modern political movements across the spectrum.
This platform records that history as reception. It does not treat the Spartan state as an object of admiration: its military capacity rested on the permanent subjection of the helots, a population held by force and against whom war was formally declared each year. See the Spartan army.