Battle of Thermopylae
The Battle of Thermopylae was a famous battle during the Greco-Persian Wars, fought in north western greece. It is known for the fact that 300 Spartan soldiers and 3700 other Greek soldiers (most notably the Thespians) held off an army of several hundred thousand Persian soldiers (led by Xerxes) for three days, therefore giving the rest of Greece time to group and prepare to the south. It was considered to be a defining moment in the war.
The battle of Thermopylae was fought between the Greeks and the Persians in 480 BC. The Persian "God king" Xerxes, master of the huge Persian Empire, was waging war on Greece for control of its lands and people. The battle is most known for two factors: 1) the impressive show of strength by the Persians, whose forces numbered between 200,000 to 2 million 2) the determination and bravery of the few thousand Greeks who tried to stop them. The estimated 6700 Non-Spartan Greeks (Athenians and soldiers from other city-states) were led by the Spartan king Leonidas. The Spartans themselves numbered a few hundred.
The Greek soldiers were able to successfully hold off the Persians because of the terrain and their fighting style. Because the mountain pass was so narrow at the location they had chosen to fight, the Greeks only had to face a small number of the Persian army at a time. Also, the Greek phalanx, a fighting style consisting of ten rows and ten columns of soldiers carrying twelve foot long spears and rounded shields gave the Greeks a huge advantage. This fighting style was so effective because the tight formation and shields left practically no part of a soldier unprotected, and the twelve foot long spears enabled men from a few rows back to attack. It was also effective because when a Greek soldier died on the front line, he was immediately replaced by a new soldier. After two days of battle in which the Persians were unable to advance, a traitorous Greek farmer named Ephialtes told the Persians of a secret path that let the Persians get around and attack the Greeks from behind. 300 Spartans and 700 Thespians stayed, while the Athenians left to gather more forces, and the impressive last stand allowed Sparta and Athens to regroup and eventually defeat the Persian forces in the Battle of Platea. It is estimated that 20,000 Persians were killed by the Greeks. The traitor Ephialtes is so derided in Greek tradition, that his name is now a synonym for "nightmare."
The battle of Thermopylae was considered to be a key battle in preventing the overrunning of Western civilization by Eastern invaders.
A movie was made about this battle in 2007 named "300". However, the movie was based upon the highly-stylized graphic novel by Frank Miller and so mixes historical fact with historical fiction. 
Cartledge, Paul, Thermopylae; the Battle that Changed the World, Overlook Press, New York, 2006