The Athenian League, also known as the Delian League, was an alliance of Greek city-states led by Athens during the Classical period. Formed in 478 BCE after the Persian Wars, its primary purpose was to provide mutual defense against potential Persian attacks and to secure trade routes. Member states contributed ships or money to a common treasury located on the island of Delos. Over time, the Athenian League transformed into an Athenian empire, as Athens exerted increasing control over its allies. This shift led to tensions and conflicts, particularly with Sparta and its allies, culminating in the Peloponnesian War (431-404 BCE). The league's decline marked a significant change in the balance of power in ancient Greece.