After the defeat of the Persians in 479 B.C., Athens dominated Greece politically, economically, and culturally. The Athenians organized a confederacy of allies to ensure the freedom of the Greek cities in the Aegean islands and on the coast of Asia Minor. Members of the so-called Delian League provided either ships or a fixed sum of money that was kept in a treasury on the island of Delos, sacred to Apollo. With control of the funds and a strong fleet, Athens gradually transformed the originally voluntary members of the League into subjects. By 454/453 B.C., when the treasury was moved from Delos to the Athenian Akropolis, the city had become a wealthy imperial power. It had also developed into the first democracy. All adult male citizens participated in the elections and meetings of the assembly, which served as both the seat of government and a court of law.

http://www.metmuseum.org/toah/hd/tacg/hd_tacg.htm