Leonidas was born around the year 530 BC and was the son of a Spartan king. Leonidas had two siblings, an older brother Dorieus and a younger brother Cleombrotus. His father was believed to be descended from Heracles. Leonidas would be given the highest Spartan honor in the Battle of Thermopylae.
The Spartan lifestyle was very different than any other Greek city-state. Instead of teaching their sons the arts of reading and writing they set up a system dedicated to producing warriors. Every male Spartan had to become a full time soldier and spent his life training and fighting. From the age of 7 they were trained for war. These boys were placed in groups of 15, the bravest of the company would be made captain. They were kept under very strict rules. The captain of the of the group would give out punishments as he saw fit. This made all Spartans tough, proud, and disciplined. By the Classical Period Sparta had become the strongest military power in all Greece, and its soldiers were famous for their bravery. However, this was achieved at the expense of her cultural development. There were now no philosophers, or artists, or anything else in Sparta, just soldiers.
In 480 BC Persian King Xerxes was planning to carry out his father's intentions to conquer Greece. To do this he created a tremendous army consisting of 1,700,000 infantrymen, 80,000 in the cavalry, and the fleet had 1,207 ships in it with 200 men on each ship. Now at this time there was a Spartan oracle which said,
- 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 bulls shall hold him,
- Strength against strength; for he has the power of Zeus
- And will not be checked 'til one of these he has consumed.
Leonidas seemed to believe that he was this king.
To stop this invasion the Greeks held counsel and resolved to send a Greek force to Thermopylae. This force consisted of 4,200 Greeks and 300 Spartans, and at its head was Leonidas. Thermopylae was a very narrow passage being only 50 feet wide. On one side there is a steep cliff face, on the other was a straight drop into the ocean. This was an ideal place for the Greeks to fight for it would make their phalanx more effective, and nullify the Persian's superior numbers.