The Delian League began as anti-Persian and became Athens' empire
In 478 BC, between about 150 and 330 Greek cities joined a confederacy on Delos under Athenian leadership to keep fighting Persia. Allies increasingly paid cash instead of ships. Athens spent the treasury on itself until league meant empire.
The alliance grew out of the Persian Wars. Cyrus the Great had seized the Ionian cities of Asia Minor soon after 550 BC, and when they revolted, Athens and Eretria helped burn Sardis in 498. Darius answered with an invasion that ended at Marathon in 490, and his son Xerxes returned in 480, only for the Greeks to smash his navy and army and then, reportedly on the same day as Plataea, destroy what remained of his fleet at Mycale near Samos.
Sparta then lost interest. Its leaders feared Athenian growth, felt the war's aims had been met and doubted the Asian Greeks could be defended for long; one Spartan king even proposed moving the Ionians to the mainland. Xanthippus, the Athenian commander at Mycale, angrily refused, since the Ionian cities were Athenian colonies. Leadership passed to Athens, and a congress on the sacred island of Delos, meeting at Apollo's sanctuary, created a new league in 478 BC. Contemporaries simply called it the Athenians and their allies.
Members swore to share friends and enemies and sank iron ingots in the sea as a sign the pact would last. Each could send ships and men or pay into a common fund, and most chose to pay. Aristides spent his remaining years assessing contributions, reportedly dying in Pontus while fixing the dues of new members. Cimon, meanwhile, forced Karystos on Euboea to join and planted Athenian settlers on conquered Skyros.
Cash from allies let Athens build up its own navy, and it grew harsh with cities that fell behind; Thucydides blamed arrears and missed service for most defections. Thasos revolted in 465 BC. In 454 Pericles shifted the treasury from Delos to Athens, a gesture that made the empire plain. Fear of that power helped start the Peloponnesian War in 431, and Lysander dissolved the league after Athens lost in 404, though a Second Athenian League later revived part of it.
Source: Delian League
