After the Persians were defeated at Platea, the Spartans did very little. However, Persia was still dangerous. Athens asked the Greek cities on the islands in the Aegean and in Anatolia to join her. The cities agreed because they were afraid of Persia. These cities formed the Delian League and Athens was their leader. Many of the cities of the Delian League had to pay Athens tribute money. Athens used the money to build many ships and the Parthenon. Sparta was still strong on land, but Athens was stronger on the sea. Several times there was war between Athens and Sparta. Then Athens decided to send many ships to Sicily to fight against the city Syracuse. Sparta sent help to Syracuse, and Athens was defeated. None of the Athenian ships came back.