The Delian League was a formal alliance between Greek City states that existed between 477 BC and 392 BC, when it was reformed into the Athenian Empire.
During the Greco-Persian Wars, Greece acted for the first time as a cohesive polity. However, following the capture of Byzantion, Sparta was eager to end its involvement in the war. The Spartans were of the view that, with the liberation of mainland Greece, and the Greek cities of Asia Minor, the war's purpose had already been reached. There was also perhaps a feeling that establishing long-term security for the Asian Greeks would prove impossible. In the aftermath of Mycale, the Spartan king Leotychides had proposed transplanting all the Greeks from Asia Minor to Europe as the only method of permanently freeing them from Persian dominion. Xanthippus, the Athenian commander at Mycale, had furiously rejected this; the Ionian cities were originally Athenian colonies, and the Athenians, if no-one else, would protect the Ionians. This marked the point at which the leadership of the Greek alliance effectively passed to the Athenians. With the Spartan withdrawal after Byzantion, the leadership of the Athenians became explicit.
The loose alliance of city states which had fought against Xerxes's invasion had been dominated by Sparta and the Peloponnesian league. With the withdrawal of these states, a congress was called on the holy island of Delos to institute a new alliance to continue the fight against the Persians; hence the modern designation "Delian League". 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.
The League intermittantly warred with Persia until 450, when a formalized treaty established a lasting peace in Asia Minor. However, this warring gave the Athenians justification to move the treasury of the League from Delos to Athens.
This, in addition to Athens role in putting down rebellions by other city states, solidified its position as head of the league. The majority of the Leagues funds went towards furthering Athenian power. Following the ostracism of Cimon in 461 BC, the League neglected its alliance with Sparta, and began allying with Spartas enemies. This sparked an emnity between Sparta and Athens, which would eventually culminate in the Peloponnesian War.
Following the defeat of the Spartans, the League grew to incorporate most of Arcadia and the majority of the democratic cities on the Peloponnesian peninsula. Athens, and hence the League, was now the single largest military power in Greece, rivalled only by the de-facto Boetian Alliance.
Following the Boetian War, Athens was effectively the capital of Greece, with nearly all city-states being forced to pay tribute or provide military assistance. Eventually, this would be formalized by the reforms of Alkaios, marking the begining of the Athenian Empire.