Origins of the Delian League and the Peloponnesian League
In 431 BC war broke out between Athens and Sparta. The war was a struggle not merely between two city-states but rather between two coalitions, or leagues of city-states: the Delian League, led by Athens, and the Peloponnesian League, led by Sparta.
The Delian League grew out of the need to present a unified front of all Greek city-states against Persian aggression. In 481 BC, Greek city-states, including Sparta, met in the first of a series of “congresses” that strove to unify all the Greek city-states against the danger of another Persian invasion. The coalition that emerged from the first congress was named the “Hellenic League” and included Sparta. Persia, under Xerxes, invaded Greece in September 481 BC, but the Athenian navy defeated the Persian navy. The Persian land forces were delayed in 480 BC by a much smaller force of 300 Spartans, 400 Thebans and 700 men from Boeotian Thespiae at the Battle of Thermopylae. The Persians left Greece in 479 BC after their defeat at Plataea.
Plataea was the final battle of Xerxes’ invasion of Greece. After this, the Persians never again tried to invade Greece. With the disappearance of this external threat, cracks appeared in the united front of the Hellenic League. In 477, Athens became the recognised leader of a coalition of city-states that did not include Sparta. This coalition met and formalized their relationship at the holy city of Delos. Thus, the League took the name “Delian League”. Its formal purpose was to liberate Greek cities still under Persian control. However, it became increasingly apparent that the Delian League was really a front for Athenian hegemony throughout the Aegean.