The summer of 429 BC brought a Peloponnesian push into northwestern Greece. Sparta and its allies aimed to knock Acarnania, Zacynthus, and Cephallenia out of the Athenian alliance, and to capture the Athenian naval base at Naupactus. The Spartan navarch Cnemus crossed the Corinthian Gulf with 1,000 hoplites and moved against the Acarnanian city of Stratus. The Acarnanians appealed to Phormio for help; he refused to abandon Naupactus. Meanwhile, the Peloponnesian fleet — forty-seven triremes — was assigned to ferry troops along the southern coast of Acarnania. Phormio's twenty ships shadowed them from the opposite shore. The Peloponnesians were not particularly worried about the Athenian squadron across the water. That was their first mistake.

The Peloponnesian commanders decided to pass through the narrow strait between Rhium (on the Peloponnesian side) and Cape Antirrhium (on the northern shore) under cover of darkness, hoping to slip past Phormio without a fight. The Athenians noticed the movement and gave chase. When the Peloponnesians emerged into the open water of the Gulf of Patras, Phormio's fleet was on them. The Peloponnesians, many of whose ships were rigged as transports rather than fighting vessels, formed a defensive circle — forty-seven triremes with prows outward, the smaller ships and the five fastest triremes clustered in the center, ready to plug any gap. It was a sound formation for an inferior fleet. Against Phormio, it was not enough.

Phormio's response was unorthodox and, on its face, reckless. He led his ships in a tightening spiral around the Peloponnesian circle, darting inward at intervals to push the defenders closer together. The tactic exposed the flanks of every Athenian ship to a potential ramming attack — any Peloponnesian vessel had only to thrust straight forward to strike. But no attack came. The Peloponnesian crews, crowded together and growing more crowded by the minute, could not execute the swift, coordinated response the moment required. Oars fouled. Ships jostled. Steersmen cursed at each other across narrowing gaps. Phormio waited. He was waiting for the wind. He knew from experience that a morning wind typically blew off the gulf at dawn, and he had timed his circling for exactly that moment. When the wind rose, the Peloponnesian circle became a chaos of colliding hulls and tangled oars. Phormio attacked.

The battle that followed was brief. The Peloponnesians, unable to maneuver and already disorganized, broke toward the southern shore. In the pursuit, the Athenians captured twelve of the forty-seven ships, along with their crews. The remaining thirty-five escaped to Cyllene, where they met Cnemus returning from his own defeat at Stratus. The double failure was a serious embarrassment for Sparta: its first coordinated amphibious campaign had collapsed on both fronts. The Corinthian commanders at Rhium — Machaon, Isocrates, and Agatharchidas — could do nothing about the asymmetry Phormio had exploited. Seamanship, local knowledge, and tactical nerve had defeated numerical strength.

The victory at Rhium did not end the fighting in the gulf. Within weeks, Sparta had assembled a new fleet of seventy-seven triremes — more than triple Phormio's force. Athens dispatched twenty reinforcements, but routed them through Crete rather than directly. Phormio's original twenty ships were left to face seventy-seven alone. What followed, at the Battle of Naupactus, was closer and more desperate than Rhium — Phormio only barely preserved Athenian command of the gulf. The two battles together define one of the most remarkable naval campaigns of the Peloponnesian War: a single admiral, in waters he knew well, holding back a larger power through audacity, patience, and an intimate knowledge of the morning wind.