Leonidas, a member of the Agaid house, succeeded his half-brother, Cleomenes 1, as king, probably in 490 BCE. He was married to Cleomenes’ daughter Gorgo.  He became king when his older half-brother Cleomenes 1 died under a violent, and mysterious circumstances in 490 BCE. During Leonidas’ famous battle at Thermopylae he and 300 other Spartans were killed.

         In summer of 480, Leonidas led an army of 6,000 men and 300 Spartans into Thermopylae to take on the Persians.

           For two days Leonidas, the Spartans, and the other allies fought the Persians and stood their ground. Leonidas' plan worked at first, but he did not realize that there was a secret path around the mountain that would lead into the back of the Greek army at Thermopylae.

            Leonidas's sacrifice unfortunetly did not prevent the Persians from moving past them down the Greek coast into Boetotia.
 
            All who stayed at Thermopylae were eventually killed by Persian archers who rained sheets of arrows down on the Greeks who were still left. Leonidas along with the 300 Spartans and allies who refused to leave were all killed by the Persians eventually. After Leonidas was shot with an arrow and died, the Spartans tried to preserve his body but died trying to. The Persians found and beheaded Leonidas' body, which was an act that was considered a grave insult to the Greeks.

            His death at Thermopylae showed how much Sparta was willing to sacrifice itself for the protection of the Greek region.

              Leonidas was honored with lasting fame even after his death at the Battle of Thermopylae. Because of Leonidas, hero cults were an established custom in ancient Greece from the eighth century and further on. 

Leonidas also claimed to be the descendant of the demigod Hercules.