The Persian War

The Persian Wars was a series of wars between the invading Persian Empire against the city-states of Greece. By 522 BC Persia held control over the Ionian Greek city-states in Anatolia. They revolted unsuccessfully in the Ionian Revolts, but was helped by Athens, one of the most powerful city-states at the time. Darius I, the King of Persia was upset with Athens, and attacked in the Battle of Marathon. The Persians were badly beat, and sent home in shame.
     In 480, Persian's new leader Xerxes I invaded Greece once again, this time seeking revenge. The most famous of these fights was the Battle of Thermopylae, where a miniscule band of a few thousand Spartans, under Leonidas courageously fought the Persian army, consisting of hundreds of thousands of men. The Spartan ended up losing, but this battle had gone down in history because of the amount of time it took the large Persian Army to defeat the small group of Spartans. Later, when Xerxes reached Athens, they were destroyed in the Battle of Salamis. In 479, Xerxes withdrew it to Persia. His army was defeated at the Battle of Plataea the same year, and driven from Greece.

This video explains basically why the Spartans where such a powerful force.