First Peloponnesian War
The First Peloponnesian War (460–445 BC) was fought between Sparta as the leaders of the Peloponnesian League and Sparta's other allies, most notably Thebes, and the Delian League led by Athens with support from Argos. This war consisted of a series of conflicts and minor wars, such as the Second Sacred War. There were several causes for the war including the building of the Athenian long walls,  Megara's defection and the envy and concern felt by Sparta at the growth of the Athenian Empire. The First Peloponnesian War began in 460 BC with the Battle of Oenoe, where Spartan forces were defeated by those of Athenian-Argive alliance. At first the Athenians had the better of the fighting, winning the naval engagements using their superior fleet.