The Delian League was an alliance made up of a number of Greek city states. The League was created in 477 BCE. The point of the League was to oppose the Persians. The Greeks had defeated the Persians in the Persian War, but the members of the League felt the need to band together to continue to resist Persia in case of renewed hostilities.
The Delian League is most important historically because of the part it played in the Peloponnesian Wars. Athens was the leading polis in the League, but it ruled the League with too heavy of a hand and caused unhappiness in some of its allies. Their unhappiness, together with Spartan fears of Athenian domination, led to the Peloponnesian Wars.