Leonidas I, born around 540 BCE, was the king of Sparta and a member of the Agiad dynasty, which claimed descent from the demigod Heracles. He was the son of King Anaxandridas II and ascended to the throne around 489 BCE, succeeding his half-brother Cleomenes I. Leonidas ruled jointly with King Leotychidas II until his death in 480 BCE, after which his son Pleistarchus became king.During the Second Greco-Persian War, Leonidas led the Greek forces at the Battle of Thermopylae in August 480 BCE, where he aimed to defend the pass against the invading Persian army. He was killed on the third day of battle, becoming a legendary figure as the leader of the 300 Spartans who fought to the death. Although the Greeks lost this battle, they successfully expelled the Persian forces the following year.Leonidas' mother was both his father's wife and niece, and after a long period of barrenness, she bore him and his brothers. His father, Anaxandridas II, died around 524 BCE, and Leonidas' half-brother Cleomenes succeeded him. Dorieus, another half-brother, was unable to accept this and sought fortune elsewhere, ultimately leading to his death in Sicily. Leonidas married Gorgo, the daughter of Cleomenes, before becoming king.Leonidas was a full citizen of Sparta during significant events, including the Battle of Sepeia against Argos around 494 BCE and the Persian demand for submission in 492/491 BCE. His elder half-brother Cleomenes was deposed and fled into exile before the First Persian invasion, which culminated in the Battle of Marathon in 490 BCE.In 481 BCE, Leonidas was chosen to lead the Greek forces against Xerxes' invasion. His selection was based on his military capabilities, as evidenced by the coalition's preference for Athenian leadership after his death. Leonidas consulted the Oracle at Delphi before marching to Thermopylae with a small force of 1,200 men, later joined by other Greek city-states, totaling around 7,000 troops.The Persian army, led by Xerxes, was significantly larger, with estimates ranging from 70,000 to 300,000 troops. After four days of waiting, the Persians attacked, but Leonidas and his forces repelled their assaults, inflicting heavy casualties. On the seventh day, a Greek traitor revealed a mountain path to the Persians, prompting Leonidas to send away most of the Greek troops to preserve their strength for future battles. He remained with 300 Spartans, 900 helots, 400 Thebans, and 700 Thespians, who chose to stay and fight alongside him.All of the remaining Greek forces were killed except for the 400 Thebans, who surrendered. Leonidas' body was retrieved by his men after fierce fighting, and Xerxes ordered his head to be severed and displayed, a sacrilegious act in Greek culture.A hero cult for Leonidas persisted in Sparta until the 2nd century CE, with annual solemnities held in his honor. His legacy continued through various cultural representations, including statues and literary works, solidifying his status as a symbol of bravery and sacrifice in the face of overwhelming odds.