The Delian League was an association of approximately 150 5th-century BC Greek city-states under the leadership 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 Greco–Persian Wars. Founded in 478 BC, League's name derives from its official meeting place, the island of Delos, where congresses were held in the temple and where the treasury stood until Pericles removed it to Athens in 454 BC.
According to Thucydides, the official aim of the League was to "avenge the wrongs they suffered by ravaging the territory of the king. In reality, this goal was divided into three main efforts - to prepare for future invasion, to seek revenge against Persia, and to organize a means of dividing spoils of war. League members swore to have the same friends and enemies, and dropped ingots of iron into the sea to symbolize the permanence of their alliance.
Thucydides indicates that this is how Athens' control over the League grew:
Of all the causes of defection, that connected with arrears of tribute and vessels, and with failure of service, was the chief; for the Athenians were very severe and exacting, and made themselves offensive by applying the screw of necessity to men who were not used to and in fact not disposed for any continuous labor. In some other respects the Athenians were not the old popular rulers they had been at first; and if they had more than their fair share of service, it was correspondingly easy for them to reduce any that tried to leave the confederacy. The Athenians also arranged for the other members of the league to pay its share of the expense in money instead of in ships and men, and for this the subject city-states had themselves to blame, their wish to get out of giving service making most leave their homes. Thus while Athens was increasing her navy with the funds they contributed, a revolt always found itself without enough resources or experienced leaders for war.
In 465 BC Thasos revolted against the Delian League. After two years Thasos surrendered to the Athenian leader Cimon. In result, the fortification walls of Thasos were torn down, their land and naval ships were confiscated by Athens. The mines of Thasos were also turned over to Athens and they had to pay yearly tribute and fines.
In 461 BC, Cimon was ostracized, and was succeeded in his influence by democrats such as Ephialtes and Pericles. This signaled a complete change in Athenian foreign policy, neglecting the alliance with the Spartans and instead allying with her enemies, Argos and Thessaly. Megara deserted the Spartan-led Peloponnesian League and allied herself with Athens, allowing construction of a double line of walls across the isthmus of Corinth, protecting Athens from attack from that quarter. Around the same time, due to encouragement from influential speaker Themistocles, they also constructed the Long Walls connecting their city to the Piraeus, its port, making it effectively invulnerable to attack by land.
In 454 BC the Athenian general Pericles moved the Delian League's treasury from Delos to Athens, allegedly to keep it safe from Persia. However, Plutarch indicates that many of Pericles' rivals viewed the transfer to Athens as usurping monetary resources to fund elaborate building projects. Athens also switched from accepting ships, men and weapons as dues from league members, to only accepting money. The new treasury established in Athens was used for many purposes, not all relating to the defense of members of the league. It was from tribute paid to the league that Pericles sent to build the Parthenon in the Acropolis, as well as many other non-defense related expenditures. The Delian League was turning from an alliance into an empire.
This was Athenians' main (public) reason for moving the treasury of the League from Delos to Athens, further consolidating their control over the League. The Persians followed up their victory by sending a fleet to re-establish their control over Cyprus, and 200 ships were sent out to counter them under Cimon, who returned from ostracism in 451 BC. He died during the blockade of Citium, though the fleet won a double victory by land and sea over the Persians off Salamis.
This battle was the last major one fought against the Persians. Many writers report that a formal peace treaty, known as the Peace of Callias, was formalized in 450 BC, but some writers believe that the treaty was a myth created later to inflate the stature of Athens. However, an understanding was definitely reached, enabling the Athenians to focus their attention on events in Greece proper.
The peace with Persia, however, was followed by further reverses. The Battle of Coronea, in 447 BC, led to the abandonment of Boeotia. Euboea and Megara both revolted, and while the former was restored to its status as a tributary ally, the latter was a permanent loss. The Delian and Peloponnesian Leagues signed a peace treaty, which was set to endure for thirty years. It only lasted until 431 BC, when the Peloponnesian War broke out.
Those who revolted unsuccessfully during the war saw the example made of the Mytilenians, the principal people on Lesbos. After an unsuccessful revolt, the Athenians ordered the death of the entire male population. After some thought, they rescinded this order, and only put to death the leading 1000 ringleaders of the revolt, and redistributed the land of the entire island to Athenian shareholders, who were sent out to reside on Lesbos.
This type of treatment was not reserved solely for those who revolted. Thucydides documents the example of Melos, a small island, neutral in the war, though originally founded by Spartans. The Melians were offered a choice to join the Athenians, or be conquered. Choosing to resist, their town was besieged and conquered; the males were put to death and the women sold into slavery (see Melian dialogue).
Athens's empire was not very stable, and only 27 years of war with the Spartans, aided by the Persians and internal strife, were able to defeat it. However, it did not remain defeated long. The Second Athenian Empire, a maritime self-defense league, was founded in 377 BC and was led by Athens; but Athens would never recover the full extent of her power, and her enemies were now far stronger and more varied.