Not much is known about the life of Leonidas and most of what we do know today comes from the Greek researcher Herodotus of Halicarnassus. Leonidas was never expected to be king, with two older brothers and their chances of producing male heirs, no one even thought it was a possibility.
Anaxandrides was a Spartan king who ruled from roughly 560 B.C. until his death in 520 B.C. During his reign, his Queen was unable to provide Anaxandrides with children and heirs for such a long period of time that the elected administrators of the Spartan constitution, called ephors, tried to convince him to set aside his Queen for another. When Anaxandrides heard of this, he refused and told the ephors that he would not set aside his Queen because it was not her fault that she was unable to bear children. The administrators knew that Sparta needed heirs for the Agiad dynasty, so they all agreed that Anaxandrides could take a second wife while still keeping his first, a very unusual practice.
Not long after his marriage, Anaxandrides’s second wife became pregnant and gave birth to a son, Cleomenes. In a terrible twist of irony, his first wife also became pregnant at the same time and would give birth to Anaxandrides’s second son, Dorieus. The first wife proceeded to provide her king with another two sons, Leonidas and Cleombrotus respectively. No other known issues are reported from his second wife.
What this meant to Leonidas was that he had two older brothers who were in line of succession before him, making his lot as king highly unlikely. Spartan tradition for rearing boys was that they stayed at home with their mothers until the age of 7 when they would then attend the agoge. Little is known about what the agoge entailed but it has been assumed that it was a rigorous place for education, learning discipline, fighting, and moral values. Future Kings of Sparta did not attend the agoge but since no one ever thought Leonidas would make it to rule Sparta, he attended the agoge thus making him one of the only Kings of Sparta who did.
When Anaxadrides died in 520 B.C., there was much debate as to who would take his place: would it be the eldest son Cleomenes from his second wife or did the second son Dorieus, from his first wife, have a better claim? It was the Spartan tradition for the first born son to succeed, so Cleomenes became king much to the dismay of Dorieus, who felt that he was the first born legitimate son. It should be noted that Spartan kings did not rule alone, there was always a co-king, and that co-king with Cleomenes was Demaratus, the Eurypontid Spartan king.
With Dorieus angry with Cleomenes’s position, he asked the Spartans to send him away to colonize northern Africa. Dorieus set out, headed south and founded a town in what is now Libya around 515 B.C. and then kept moving west to Sicily. What happened next was never properly recorded but we do know that in a town near Sicily, called Heraclea, an alliance of Segestans and Carthaginians came to cease Spartan colonization. The Spartans lost the battle resulting Spartan retreat and Dorieus’s death. Dorieus never produced a male issue during his life.
Meanwhile, back in Sparta, Leonidas was now the heir to his unlikely rule and things were beginning to get complicated for Cleomenes, who still had no produced an heir. Starting about 493 B.C., Persian forces began invading Greece and Greek city-states were submitting to the Persians out of fear of losing any trade business. This Persian force kept invading and in 491 B.C. Cleomenes, as a means to stop them, tried to arrest the major Persian collaborators in these areas. Remember Demaratus, the co-king? He was not pleased with Cleomenes’s actions and tried to stop him from making a huge mistake. Cleomenes then did what any mentally unstable person, as Dorieus called him, would do: he overthrew Demaratus who deserted to the Persian Empire. With the threat to his rule gone, Demaratus found someone to replace Demaratus who would agree with him, King Leotychides. The two kings working together did capture the Persian collaborators but 3 years later things became a little more difficult for the Spartan king.
In or about 490 B.C., Cleomenes had to flee from Sparta when it was discovered what he had done to Demaratus. Cleomenes was allowed to return to Sparta not long after his departure and this is when Herodotus mentions Cleomenes’s insanity for the first time. It was all a ruse though, the Spartans promptly arrested him, and under the orders of Leonidas and Cleombrotus, Cleomenes was also forced to be put in chains while in prison. It was while he was locked up that Cleomenes was found dead from an apparent suicide by self-mutilation.
Now we have come to Leonidas as king of Sparta. Cleomenes had still not produced a male heir by the time of his death, leaving Leonidas as the rightful heir and became king sometime between 487 B.C. and 490 B.C. Leonidas had married his half-niece Gorgo, the daughter of Cleomenes, which resulted in the birth of their son, Pleistarchus. When the Spartans battled Xerxes and the Persians at the Battle of Thermopylae in August of 480 B.C., Leonidas, his faithful 300, 700 Thespians and 900 Helots, had all lost their lives. These 1,900 men refused to leave the side of Leonidas to face a force of an estimated 70,000 to 300,000 Persians in the famous battle. Leonidas’s son succeeded his father as king of Sparta when he came of age to rule on his own.