This clash was the opening sea fight of the Roman-Seleucid War, a collision between the rising power of the Roman Republic and the sprawling empire of Antiochus III the Great, who ruled lands from the Aegean to the borders of India. Antiochus had pushed into Asia Minor and across into Europe, alarming Greek cities that appealed to Rome for protection. When Rome and Antiochus failed to settle their differences, war came. By the spring of 191 BC the Seleucid king had already been beaten on land at Thermopylae and had withdrawn to Ephesus. The struggle now moved to the water, where the Romans needed control of the Aegean to carry the fight into Asia Minor itself.

Antiochus handed his fleet to Polyxenidas, an exile from Rhodes serving the Seleucid crown, with roughly seventy decked warships and more than a hundred lighter galleys. Against him came Livius with eighty-one Roman quinqueremes, a contingent of Carthaginian and Pergamene vessels, and the promise of a Rhodian squadron still gathering at Samos. Polyxenidas chose his ground shrewdly, taking position at the harbor of Kissos, southwest of Corycus, where he could intercept the allies sailing in either direction. What he did not have was local goodwill. The people of Chios fed Livius a steady stream of intelligence on Seleucid movements, and that knowledge let the allied fleet unite its forces before the trap could close.

Once the lines met, the fighting turned on Roman strengths. Where the Seleucids relied on lighter, faster ships, the Romans clamped on with grappling hooks and turned each contact into a boarding action, sending marine infantry across the rails to overwhelm the enemy's thinner crews. A Roman squadron on the right wing swung landward to strike the Seleucid flank from the side. Polyxenidas, watching his formation buckle, broke off and ran for Ephesus. He lost twenty-three ships in all, ten sunk and thirteen captured with their crews. The allies, their decks still loaded with supplies, lost a single vessel and did not pursue. It was a lopsided result, and a telling one.

Corycus did not end the war, but it set the pattern for what followed. With the Seleucid fleet bottled toward Ephesus, the allies took the initiative at sea. Antiochus, undeterred, summoned Hannibal himself to build a new fleet in Phoenicia, the famed Carthaginian general's first real command in years of exile. Reading the lesson of Corycus, Hannibal bet on large, heavy vessels to resist Roman boarding tactics. It would not be enough. Within two years the Rhodians shattered one Seleucid fleet at the Eurymedon and the allies broke another at Myonessus, clearing the way for the Roman army to cross into Asia and end Seleucid ambitions at Magnesia. The turning had begun here, off this quiet cape.