Phormio first appears in the historical record in 440 BC, when he shared with Thucydides, Hagnon, and others the command of the Athenian fleet in the later part of the Samian War.
[6] In the summer of 429 BC, however, Sparta began preparing a sizeable fleet and army to attack Athens' allies in the region, hoping to overrun Acarnania on land, capture the islands of Zacynthus and Cephallenia, and possibly even take Naupactus.
The tactic was a risky one—it left the Athenians' flanks utterly vulnerable to ramming—but it paid off when a wind blew up and caused the inexperienced crews of the circled vessels to foul their oars.
This victory preserved Athenian naval supremacy in the Gulf and put an end to Peloponnesian attempts to challenge it during this period of the war.
An Athenian defeat in the Corinthian Gulf in 429/8 BC would have been a devastating blow to Athens' influence in the Greek north-east, and to the city's reputation for naval invincibility.