Phormio, one of the generals who had led reinforcements to the Athenian siege of Samos in 440, assists the Acarnanians and Amphilochians against Ambracia around the same time.

This results in an alliance with Acarnania, a place close to areas of traditional Corinthian influence.

Potidaea revolts and an unofficial Corinthian force goes out to help.

Potidaea is laid under siege by Athens, with Phormio in command.

Alcibiades, well-born and wealthy, had been only a small boy when his father—who was in command of the Athenian army—was killed in 447 or 446 BCE, at Coronea in Boeotia, and his guardian, Pericles, a distant relation, has been too preoccupied with political leadership to provide the guidance and affection that the boy needed.

Alcibiades has grown up to be a strikingly handsome and keen-witted teenager, but he is extravagant, irresponsible, and self-centered as well.

He is, however, impressed by the moral strength and the keen mind of the philosopher Socrates, who, in turn, is strongly attracted by Alcibiades' beauty and intellectual promise.

They serve together at Potidaea, where Socrates defends Alcibiades when the latter is wounded.

Potidaea has surrendered, and a cleruchy is installed here too, a further Corinthian setback, in 430 BCE.

Phormio, the Athenian general who had headed the siege of Potidaea, is sent to block the entrance to the Gulf of Corinth with twenty ships in 430 BCE.

Athens finds more concrete encouragement in some naval successes of the great commander Phormio in the Gulf of Corinth.

His twenty ships positioned at the entrance to the gulf, Phormio wins two brilliant victories in the summer of 429.

In the first, he defeats forty-seven Peloponnesian ships that are advancing to reinforce the Spartan Cnemus's campaign in Acarnania; in the second battle, he routs Cnemus's seventy-seven-vessel fleet.