The battle of Naupactus (429 BC) was a second Athenian naval victory won in a short period around the Gulf of Corinth, but was won by a very narrow margin and only after the narrow failure of a Peloponnesian plan to trap the entire Athenian fleet (Great Peloponnesian War).
In 429 BC the Athenians had a small squadron of twenty triremes posted in the Gulf of Corinth under the command of Phormio. He soon found himself facing a much larger Peloponnesian fleet, sailing west along the Gulf of Corinth to join an army that was invading Acarnania, the area to the north west of the entrance to the gulf. Phormio intercepted this fleet just outside the fleet, and defeated it at the battle of Chalcis, capturing (and probably destroying) twelve of the enemy ships.
The surviving Peloponnesian warships escaped west to Cyllene, at the north-western tip of the Peloponnese, where they were joined by the fleet that had taken part in the unsuccessful invasion of Acarnania. There the fleet received reinforcements, as well as an advisory commission sent out from Sparta (including the famous general Brasidas). When the combined fleet finally put back out to sea it contained seventy seven warships. The Athenians also sent out reinforcements, but they went via Crete, where they were delayed for some time, so Phormio had to fight this second battle with the same twenty ships as the first.
The two fleets anchored on opposite sides of the Gulf of Rhium, with the Athenians at Molycrian Rhium on the northern side of the gulf and the Peloponnesians at Rhium in the Peloponnese. For six or seven days the two fleets faced each other across less than a mile of sea. According to Thucydides the sailors in both fleets suffered from poor morale during this period - the Peloponnesians because of their recent defeat and the superior seamanship of the Athenians, the Athenians because they were outnumbered by nearly four-to-one. Thucydides uses this as an opportunity to include speeches made by the commanders of the respective fleets, in which he had the Spartan commanders emphasising their superior courage, while Phormio spoke about his men's equal courage and superior skill. He also promised not to fight in the narrow waters of the Gulf of Rhium, where the enemy's superior numbers could be decisive.
Despite this promise the Peloponnesian strategy soon forced Phormio to sail straight into those narrow waters. The Peloponnesian fleet put to sea and formed up into four lines, each parallel to the coast. The twenty fastest ships were posted at the right of these lines. The fleet then sailed east along the coast in this formation, with the army following on shore. Phormio was immediately faced with a dilemma. If he didn't counter the Peloponnesian move then they would be able to capture his base at Naupactus, but if he did move then he would soon be forced into the narrow waters. The threat to Naupactus was too serious to be ignored, and the Athenian fleet put to sea and sailed east in single file, also with their army following on the coast.
Once both fleets were inside the Gulf of Rhium the Peloponnesian commanders attempted to close their trap. Their entire fleet turned north and headed straight for the Athenians, hoping to pin the entire Athenian fleet against the northern shore of the gulf. This move was only a partial success. Eleven of the twenty Athenian ships managed to escape from the trap and made it into the wider waters of the Gulf of Corinth. The remaining nine were forced onto the shore. The crews of most ships either swam ashore or were killed in the attempt. One was captured with its entire crew, others were captured empty and towed away, while some were rescued by the Athenian's allies on the northern shore.
While the Peloponnesians were apparently winning a major victory in the gulf, their twenty fastest ships were engaged in a pursuit of the remaining eleven Athenian ships. In both fleets one ship came adrift from the others. Ten of the Athenian ships reached Naupactus, where they took up defensive positions, with their prows pointing out towards the pursuing enemy. The last was trailing somewhat behind. In the Peloponnesian fleet one ship, from the island of Leucas, got ahead of the other nineteen.
As these two isolated ships approached Naupactus, the Athenian ships swung around an anchored merchant ship and rammed the leading pursuer, sinking it. This encouraged the other Athenian ships to rejoin the battle. The surviving nineteen Peloponnesian ships suddenly found themselves facing eleven attacking Athenian ships. The Peloponnesian squadron had lost any semblance of a formation during the pursuit, and was quickly routed. The Athenians then pursued the defeated Peloponnesians, capturing six of them, and freeing all but one of the ships they had lost in the first stage of the battle.
In the aftermath of the battle both sides raised a trophy to claim a victory - the Peloponnesians for the battle in the gulf and the Athenians for the counterattack close to Naupactus. The Peloponnesians still outnumbered the Athenians, but after two defeats their confidence was low, and fearing that reinforcements would soon arrive they sailed east and took refuge at Corinth. Soon after this Athenian reinforcements did arrive, in the shape of the twenty ships that had been sent via Crete, and for the moment the Athenians controlled the Gulf of Corinth.