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".

The city of Athens during the classical period of ancient Greece was the major urban centre of the notable polis (city-state) of the same name, located in Attica, Greece, leading the Delian League in the Peloponnesian War against Sparta and the Peloponnesian League. Athenian democracy was established in 508 BC under Cleisthenes following the tyranny of Isagoras. This system remained remarkably stable, and with a few brief interruptions remained in place for 180 years, until 322 BC. The peak of Athenian hegemony was achieved in the 440s to 430s BC, known as the Age of Pericles.