In 429 BC, the Spartan government had ordered the navarch Cnemus to launch an offensive in the Corinthian gulf and northwestern Greece aimed at knocking several Athenian allies out of the Peloponnesian War.

[2] The Spartan government, dismayed by this unexpected reversal, dispatched several advisors (including the aggressive and talented Brasidas) to oversee Cnemus as he resumed the offensive.

[3] Phormio, meanwhile, sent to Athens for reinforcements, and twenty ships were sent out to him; these were ordered to first sail to Crete to attack Cydonia, and as a result did not arrive in time to participate in the battle.

[6] With the fleets at anchor across the strait from each other, it fell to the Peloponnesians to initiate a battle, as they wished to force an action before the Athenian reinforcements arrived.

[8] The Athenians, who in the previous battle had been able to take advantage of their superior maneuverability in the open sea, were reluctant to enter the constricted waters of the Gulf, but were forced to follow the Peloponnesians in order to protect Naupactus, which had been left ungarrisoned.

On the right of these lines, leading the advance into the gulf, were the twenty best and fastest ships in the fleet, which had been assigned the job of preventing the Athenians from escaping when the Peloponnesians attacked.

At Naupactus, the Athenians' backs had been against the wall; a defeat there would have lost Athens its foothold in the Corinthian gulf and encouraged the Peloponnesians to attempt further aggressive operations at sea.

[10] In 428 BC, the Spartans would be so cowed by the memory of their defeats at Phormio's hands that they sent no aid to Athens' rebellious subject Mytilene in that year.