The Delian League was an alliance of Greek city-states formed in 478 BCE after the Persian Wars. Its main purpose was to protect against future Persian attacks and to secure freedom for Greek cities. The league was named after the island of Delos, where its treasury was initially located. Members contributed money or ships, and the league was led by Athens, which became a powerful naval force. Over time, the Delian League transformed into an Athenian empire. Many member states felt pressured to follow Athenian policies, leading to resentment. This shift contributed to tensions that eventually sparked the Peloponnesian War between Athens and Sparta.