The cause of the Persian War was because the Greek colonies of Ionia on the coast of Asia Minor were conquered by the Persians under Cyrus the Great. The Greek colonies had been founded with the full rights and privileges of an independent city-state and were independent from even the Greek city-state from which the colonists came. However, Darius decided to raise the taxes the Ionian city-states needed to pay to Persia. After the Persian threat was gone, several Greek city-states formed the Delian League to protect the Greeks from any future foreign invasions. Persians had lost the war, and were weakened because of the lack of leadership. As for Athens, they needed to be rebuilt. After the war, Athens became one of the most powerful city-states.
