After Byzantium, the Spartans were allegedly eager to end their involvement in the war. The Spartans were supposedly 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 securing 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 marks the point at which the leadership of the Greek Alliance effectively passed to the Athenians. With the Spartan withdrawal after Byzantium, the leadership of the Athenians became explicit.

The loose alliance of city-states that 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. This alliance, now including many of the Aegean islands, was formally constituted as the 'First Athenian Alliance', commonly known as the 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 members were given a choice of either supplying armed forces or paying a tax to the joint treasury; most states chose the tax.