In 429 BC, several ambitious Spartan expeditions in western Greece had come to grief at the hands of Athenian naval power. The past winter, a small Athenian fleet of 20 triremes under Phormio had arrived in Naupactus to blockade the Gulf of Corinth. Early in 429, the Spartan general Cnemus managed to slip across into Acarnania with 1,000 hoplites to help an Ambraciot army attack the city of Stratus. The Corinthians sent out a fleet of 47 triremes to reinforce him, which was intercepted by Phormio's 20 ships. The Peloponnesians showed little confidence in their naval ability. Despite their numerical superiority, the Corinthians adopted a purely defensive formation, forming their ships into a circle with the prows facing outward. The Athenians formed a line and began sailing around them, threatening to ram at every pass. A wind soon came up, as Phormio knew it did every morning, and the Corinthian ships began to lose formation and drift into each other. When the confusion was at its height, Phormio gave the signal to attack, and the Corinthian fleet was routed with great loss. The Spartan reaction to this debacle is worth quoting in Thucydides' own words:
The Spartans now sent out to Cnemus and his fleet an advisory commission . . .. Their orders were to make ready for another battle at sea and to do better in it and not to be driven off the sea by a few ships. For the Spartans, especially since this was their first taste of naval engagements, found it very difficult to understand what had happened, and so far from thinking that there was anything wrong with their own navy, concluded that the defeat was the result of cowardice, not taking into consideration the contrast between the long experience of the Athenians and the short training which their own crews had received.
Phormio meanwhile sent word to Athens for reinforcements, and twenty more ships were dispatched. Before they arrived, however, the second battle took place. The Peloponnesian fleet, reinforced to 77 ships and under the joint command of Brasidas and Cnemus, faced off against the Athenians at the mouth of the Gulf of Corinth for a week. The Athenians wanted the Peloponnesians to sail out into the wider channel where their seamanship could come into play; Brasidas and Cnemus, for the same reasons, wanted the Athenians to come into the narrower waters. Finally, the Peloponnesians forced the battle on their own terms by sailing towards Naupactus. Phormio was forced to defend his base. Nine of his ships were trapped and driven ashore. The remaining 11 Athenian triremes were pursued to Naupactus by the now-disordered Peloponnesian fleet, already singing the victory chant. Suddenly, the trailing Athenian ship turned about and rammed its closest pursuer. At this, the leading Peloponnesian ships stopped to let the rest of the fleet catch up with them, and the remaining Athenian ships pounced, capturing 6 enemy ships and scattering the rest in confusion. The Athenians also recovered their own ships which had been taken in the first part of the battle. Fearing the arrival of Athenian reinforcements, the Peloponnesian fleet retired to Corinth.
The Peloponnesians were not quite finished. Upon learning that the Megarans had 40 triremes laid up in Nisaea, the Peloponnesian commanders decided on an audacious plan. The Athenians, overly confident in their own naval superiority, had left the harbor of Piraeus open and unguarded. Taking their crews across the Isthmus of Corinth, the Peloponnesians manned this new fleet and left Nisaea by night with nothing to stop them from sailing into Piraeus and possibly ending the war at one stroke. But then their nerve failed, and instead they contented themselves with plundering the small island of Salamis off the Attic coast. The news that a Peloponnesian fleet was at Salamis touched off a panic in Athens, but when they manned their ships and sailed out the next morning, the Peloponnesians retired without a fight.