Leonidas ( , Greek: ; "Lion's son", "Lion-like") 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. While it has been established that King Leonidas of Sparta died at the Battle of Thermopylae in August, 480 BC, very little is known about the year of his birth, or for that matter, his formative years. Paul Cartledge has narrowed the date of the birth of King Leonidas to around 540 BC. If it is assumed that Leonidas was born anywhere in the years subsequent to 540 B.C., this would have placed him in the 50+ year old range at the time of the conflict with the Persians.

Leonidas was one of three brothers: he had an older brother Dorieus and a younger brother Cleombrotus, who ruled as regent for a while on Leonidas' death before the regency was taken over by Pausanias, who was Cleombrotus' son. Leonidas succeeded his half-brother Cleomenes I, probably in 489 or 488 BC, and was married to Cleomenes' daughter, Gorgo. His name was raised to heroic status as a result of the events in the Battle of Thermopylae, one of the most famous battles in ancient history.

Thermopylae

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, 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. This force was assembled in an attempt to hold the pass of Thermopylae against a massive Persian army of between 80,000 and 290,000 men-at-arms who had invaded from the north of Greece under Xerxes I. Leonidas took only his personal bodyguards, and not the army, because the majority of the Spartan Army was coordinating with the massed naval forces of the Greeks against the Persian Navy. This is contrary to the belief that the army could not be sent because of religious restrictions.