A voluntary alliance formed by the Greek city-states in 478–447 bc to seek revenge for losses suffered during the Greek-Persian wars. All members paid tribute in the form of ships or money, the latter being stored on the sacred island of Delos, the League’s nominal base. At first, under the leadership of Athens, the League actively sought to drive Persian garrisons out of Europe and to liberate the Greek cities of Asia Minor. Pericles encouraged the conversion of the alliance into the beginnings of the Athenian empire.