The Delian League was a confederacy of Greek city-states, numbering between 150 and 330, founded in 478 BC under the leadership (hegemony) of Athens, whose purpose was to continue fighting the Persian Empire after the Greek victory in the Battle of Plataea at the end of the Second Persian invasion of Greece. The League functioned as a dual—offensive and defensive—alliance (symmachia) of autonomous states, similar to its rival association, the Peloponnesian League. The League's modern name derives from its official meeting place, the island of Delos, where congresses were held within the sanctuary of the Temple of Apollo; contemporary authors referred to the organization simply as "the Athenians and their Allies".
While Sparta excelled as Greece's greatest power on land, Athens turned to the seas, becoming the dominant naval power of the Greek world. Following Sparta's withdrawal from the conflict with Persia, Athens took the lead of the Hellenic alliance accompanied by several states around the Aegean and the Anatolian coast. The Delian League was formed as an anti-Persian defensive association of equal city-states seeking protection under Athens, as the latter wished to extend its support towards the Ionian Greek colonies of Anatolia. The alliance held an assembly of representatives in order to shape its policy, while members swore an oath of loyalty to the coalition. By the mid-fifth century BC, it had developed into a naval imperial power, conventionally called the Athenian Empire, where Athens established complete dominion and the allies became increasingly less autonomous. The League successfully accomplished its principal strategic goal by decisively expelling the remaining Persian forces from the Aegean. As a result, Persia would cease to pose a major threat to Greece for the following fifty years.
