The Delian League was founded in 478 BCE following the Persian War to be a military alliance against any enemies that might threaten Ionian Greeks. It was led most notably by Athens, who protected all members unable to protect themselves with its massive and powerful navy.
The End or is it?
The League was founded so that Sparta might protect itself against both a possible uprising of Sparta's helots and regional rival Argos.
The First Peloponnesian War began in 460 BC with the Battle of Oenoe, where Spartan forces were defeated by those of Athenian-Argive alliance. The Athenians were defeated in 454 BC by the Persians in Egypt which caused them to enter into a five years' truce with Sparta.
The reasons for this war are sometimes traced back as far as the democratic reforms of Cleisthenes, which Sparta always opposed. However, the more immediate reason for the war was Athenian control of the Delian League, the vast naval alliance that allowed it to dominate the Mediterranean Sea.