Sparta was a city state in Greece in ancient times. Renowned for its military prowess, Sparta was one of the most powerful cities in Greece at the time. At birth, children born in Sparta would be carefully inspected. If they were found to be wanting in any way, if they were perhaps a bit too small, or a bit sickly, they were discarded. Sparta would have only the best as its citizens. Boys began training to fight almost as soon as they could walk. Their fathers were often the ones to instruct them at this early age. At the age of eight, boys were taken from their mothers and their homes and sent to train to fight, and girls were also trained so that they would be able to bear strong boys to be great men. Boys also learned how to act together as a team and how to survive under hard circumstances. After they had passed a test to prove their strength and readiness, they would return to their homes as men wearing the scarlet of Sparta.
From an early age, Leonidas underwent training to make him into a soldier. This included being thrown out of the city in the company of other boys of his age to see if they could survive. In the graphic novel 300, Leonidas is seen killing a wolf by drawing it into a crevice and stabbing it with a wooden spear. This earns him the right to wear a red cloak, the sign of a Spartan warrior.
Later on in his life, Leonidas becomes one of the two kings of Sparta. There were two kings, one ruled in war season and the other in peace, to ensure a democratic element in the city and to make sure that there was always a leader, even if a king fell in battle.
When the Persian Emperor Xerxes invaded northern Greece, he sent a messenger to Leonidas asking for his allegiance. Leonidas and some Spartans kill the messenger for his rudeness and for suggesting that the Spartans surrender. Now Leonidas knows that war is coming.
Following Spartan law, Leonidas goes to consult the Eupors as to the course of action he has decided to take. However, they have been bribed by the Persians to tell the king that he cannot leave to fight due to a festival. Leonidas is angered by this but comes up with a solution. The next morning he straps on his sandles and makes to leave for battle. The council of elders confronts him and says that he cannot go to war because they haven't talked about it and decided all together what is to be. Leonidas tells them that he is going for a walk to the north and the army that is with him there is nothing more than his personal guards. In the comics, these are 300 childless men but this is not historically accurate. The real 300 were men with children who would secure their legacy.
On Leonidas's departure, his wife utters the famous line,
"Spartan, come back with your shield, or come back on it."
This emphasizes the importance of the shield to the Spartans. A warrior who lost his helmet and armor received no punishment, but a man who lost his shield was in serious trouble. This was due to the Spartan style of fighting called a phalanx. Every man's shield covered his friend's right side from thigh to neck, creating an impenetrable wall with spears thrust outwards.
Also, surrender and defeat were not acceptable by Spartans terms. His wife's saying also proves that in Sparta, it is better to not come back at all than face defeat. Only with a victory would Leonidas's shield return with him, for in defeat, he would be stripped of anything useful to the invading army. A bronze shield would indeed be very valuable.
On arriving at the battlefield, a place called Thermopylae, Leonidas and his men pick the place where they will set up their defense. The Hot Gates was a pass between the mountains. If Leonidas could funnel the Persians into this area, their numbers would count for nothing. Thus, a wall was built which forced any attackers into the Hot Gates. When Xerxes attacks with his lightly armed Persians, they are unable to force the Spartans back despite their massive numerical superiority.
The first day of fighting ends well for the Spartans, and so Xerxes send in his elite troops, the Immortals, so called because their unit was always kept at one thousand men strong. They were Xerxes's personal body guard and highly trained. After a bloody battle, the Spartans are once again victorious. The second day passes in much the same way. It seems as if the Spartans will succeed.
However, Leonidas is betrayed. A traitor shows Xerxes a secret pass through the mountains used by goat herders that allows the few remaining Immortals to get behind the Spartans. The few Greek guards are slaughtered, and by morning, Leonidas is surrounded.
Earlier on, he had sent a messenger back to Sparta to tell them the tale of the defenders and to psyche them up for war. He then turned to his men and said, "Make breakfast and eat hearty, for tonight we dine in hell!"
This prophecy proves accurate when Xerxes gives Leonidas a final chance to surrender. He drops to his knees. He removes his helmet. He drops his shield. It appears he is giving in. Suddenly, one of his men kills the messenger standing between him and the emperor. The Persian troops begin to shoot the Spartans down with arrows, but Leonidas manages to throw his spear. His spear slices open Xerxes's cheek, wounding the supposedly divine Emperor and proving that even a god can bleed.
Frank Miller said that his inspiration for writing the comic was the film "The 300 Spartans" that had made a big impression on him as a child.