What did themistocles do that was so important?

Themistocles was a great Athenian statesman and general who played an important role in the Second Persian War by leading the Greeks to victory. Born to an Athenian father, Neocles, and what seems to have been a foreign mother, Themistocles demonstrated great potential from an early age. 

He is said to have spent his leisure time in youth composing and performing mockspeeches, unlike other children who remained idle or engaged in play. An early teacher of Themistocles told him the following: "there is going to be nothing insignificant about you; somehow or other you will become a great man, either for good or for evil".

With much determination Themistocles strove for greatness in action and longed to distinguish himself from others, both politically and militarily. Themistocles also fought the Persians at the Battle of Marathon, and while most Athenians were convinced that the victory at Marathon would keep the Persians at bay, he believed otherwise.