The battle of Chalcis (429 BC) was the first of two Athenian naval victories won in the same year in the Gulf of Corinth that helped demonstrate their naval superiority in the early part of the Great Peloponnesian War.
In 429 the Spartans decided to launch an invasion of Acarnania, the area to the north-west of the entrance to the Gulf of Corinth. The plan was for their fleets to unite at Leucas, an island to the north-west of the gulf. Part of the combined fleet was to come from the Peloponnese and from other areas outside the gulf, while the rest sailed from Corinth and other areas inside the gulf. The first part of the fleet reached Leucas without any problems, but the forty seven ships sailing from the Corinth end of the gulf had to get past an Athenian naval force.
This small naval force, of twenty triremes under the command of Phormio, was based at Naupactus, an Athenian base on the northern shore of the gulf, just to the north-east of the narrow Strait of Rion, the western end of the gulf (the area outside the straits is called the Gulf of Rion or Rhium).
Although Phormio's twenty ships were badly outnumbered by the forty seven Peloponnesian ships, he had three advantages. First, his fleet was a single united force with a single commander, while the Peloponnesian fleet was made up of several allied contingents, each with at least one commander - the Corinthian contingent alone had three commanders: Machaon, Isocrates and Agatharchidas. Second, the Athenian fleet was manned by experienced sailors who had spent some time operating together in the Gulf of Corinth, while the Peloponnesian crews were less experienced. Finally many of the Peloponnesian ships were fitted out to transport troops and not as warships.
Phormio shadowed the Peloponnesian fleet as it sailed west along the Gulf of Corinth. On the evening before the battle the Peloponnesians moored at Patrae, on the northern coast of Achaea. In an attempt to slip past the Athenians they set sail for the north coast of the gulf of Rion during the night, but they were spotted, and the Athenians moved to intercept them.
The Peloponnesian commander's lack of faith in their fighting abilities is clearly demonstrated in the tactics they decided to adopt. Despite outnumbering the Athenians by more that two to one, they adopted an entirely defensive position, forming their fleet into a circle, with the ship's bows pointing outwards and the sterns inwards. The gap between each ship was made as wide as possible while making sure that the Athenians couldn't sail between them. Five of the fastest and best warships and all of the lighter ships were placed inside the circle, to act as a mobile reserve.
Phormio decided to try and take advantage of both the local weather conditions and the superior skills of his sailors. He knew that a strong wing normally blew early in the mornings in the Gulf of Rion, and that this wing would probably disrupt the Peloponnesian formation. In order to make it as difficult as possible for the Peloponnesians he ordered his ships to sail in circles around the enemy fleet, occasionally dashing inwards as if they were about to attack. This was something of a gamble - his hope was that the Peloponnesians would attempt to back away from his ships in an attempt to avoid battle and stay in formation, but if they had instead decided to attack then his entire fleet could have been rammed amidships.
Phormio's gamble paid off. As the Athenians circled around them the Peloponnesians backed off, reducing the size of their circle and the gaps between the ships. When the wind began to blow their ships began to run into each other. At this point Phormio finally ordered his ships to turn in and attack. One of the enemy admiral's ships was sunk first, and after that the Peloponnesian formation broke up in confusion, and each ship attempted to flee back to Patrae or to nearby Dyme. According to Thucydides the Athenians 'destroyed every ship that they came across'. He also states that they captured twelve ships along with most of their crews, but this may refer to the same ships, as Phormio still only had twenty triremes at his disposal at the battle of Naupactus, which followed soon afterwards.
In the aftermath of their victory the Athenians erected a trophy on the headland of Rhium, and then returned to their base at Naupactus. The surviving Peloponnesian ships sailed west to the dockyard at Cyllene, at the north-western tip of the Peloponnese opposite Cephalonia. There they were joined by the fleet that had taken part in the unsuccessful invasion of Acarnania, which had ended after defeat at Stratus. By the time the Peloponnesian fleet put back to sea it contained 77 triremes, while Phormio had yet to be reinforced, but despite this massive numerical inferiority the Athenians still won the next naval clash, at Naupactus.