Leonidas I was a Greek king of the Greek city-state of Sparta, and the 17th of the Agiad line, a dynasty which claimed descent from the mythological demigod Heracles. Leonidas I was a son of King Anaxandridas II. He succeeded his half-brother King Cleomenes I to the throne in c. 489 BC. His co-ruler was King Leotychidas. He was succeeded by his son, King Pleistarchus.

Leonidas had a notable role in the Second Greco-Persian War, leading the allied Greek forces to a last stand at the Battle of Thermopylae (480 BC) while attempting to defend the pass from the invading Persian army. He died during the battle and entered myth as the leader of the 300 Spartans. While the Greeks lost this battle, they were able to expel the Persian invaders in the following year.

The Spartan army stood at the center of the Spartan state, citizens trained in the disciplines and honor of a warrior society. Subjected to military drills since early manhood, the Spartans became one of the most feared and formidable military forces in the Greek world, attaining legendary status in their wars against Persia. At the height of Sparta's power – between the 6th and 4th centuries BC – other Greeks commonly accepted that "one Spartan was worth several men of any other state."

Tradition states that the semi-mythical Spartan legislator Lycurgus first founded the iconic army. Referring to Sparta as having a "wall of men, instead of bricks," he proposed reforming the Spartan society to develop a military-focused lifestyle following "proper virtues" such as equality for the male citizens, austerity, strength, and fitness. Spartan boys deemed strong enough entered the agoge regime at the age of seven, undergoing intense and rigorous military training. Their education focused primarily on fostering cunningness, practicing sports and war tactics, and also included learning about poetry, music, academics, and sometimes politics. Those who passed the agoge by the age of 30 achieved full Spartan citizenship.