The Delian League was formed by Athens in 478 B.C. to protect the Greek city-states against the Persian Empire. The representatives of the various city-states met annually at Delos, where the League's treasury was kept. However soon relations between Athens and Sparta, the other strong city-state deteriorated. Over time Athens itself became a tyranny. The League became an Athenian Empire. After the Persian threat diminished Athens became aggresive. The Athenians transferred the treasury to Athens in 454 B.C. It became obligatory to join the League. It was not allowed to secede from the League. A Thrasian revolt was brutally crushed in 463 B.C. Tribute money from the various city-states was used to rebuild temples in Athens. When Athens went to war with the Peloponnesian League, led by Sparta, the other city-states were forced to contribute men, money and ships. I hope that this has been a help to you.