Fighting Through
The Battle of Thermopolylae was one of perseverance, or at least it should have been. It is told in this book that the army of Spartans that only had about three hundred people in it defeated two million Persians. This, of course, is impossible, but it shows the fighting spirit of the Spartans. It was instilled in Spartan youth that you must fight to live, and this is a very important theme of the novel - fighting through even when things seem impossible. You will eventually win.
Faith
The Persian warriors took it for granted that they were going to win against the Greek army. They had already won many battles before this one, and were the largest land empire at the time. However, the Greeks had to have faith that they would win against this large army. Because of this faith, and their physical ability, they were able to win the battle.
Technology
At this time, and even today, technology plays a major part in wars. With the best technology, you will be able to win. This inspires both sides of the battle to improve their technology. The Spartans were able to defeat the Persians with their superior metal shields and their bow and arrows. These simple technologies were not taken for granted at the time, and the Persian army really had no idea what was coming.
