To understand 1241, it helps to know what Constantinople had become. The city had been seized and looted by crusaders in 1204, and a Latin Empire — a Western, Catholic state installed by the Fourth Crusade — now held it. But the Latin Empire was weak, financially fragile, and dependent on Venetian naval support. The Empire of Nicaea, centered in western Anatolia, was the most powerful of the Greek successor states that formed after 1204, and it aimed to reclaim Constantinople. Its emperor, John III Vatatzes, was an energetic ruler who had spent decades building up Nicaean power, picking off Latin and Venetian holdings across the Aegean and in Thrace. By 1241 his army was campaigning actively in the region, and his fleet had been deployed to support those operations.

In early 1241, John III was pursuing operations on multiple fronts simultaneously. While the Latin Empire besieged the Nicaean fortress of Tzouroulos in eastern Thrace, John led his army northward into territory above Nicomedia on the Asian side, capturing the fortresses of Dakibyza and Niketiatou. His fleet sailed in support of the army. At the same moment, a Venetian fleet sortied from Constantinople's harbor. The two fleets met off the city's sea walls. What the sources agree on is that the Nicaean fleet suffered a heavy defeat. What they disagree on is almost everything else.

The two main sources for the battle give wildly incompatible accounts. The Byzantine historian George Akropolites, writing closer to the events and generally considered reliable, recorded that the Nicaean fleet numbered 30 galleys, the Venetian fleet 13. Of those 30, the Nicaeans lost 13 ships captured — meaning each Venetian vessel took a Nicaean trireme as a prize, with its crew and weapons. The Venetian chronicler Martin da Canal, by contrast, claimed the Nicaean fleet numbered no fewer than 160 ships of various sizes, with only 10 Venetian galleys opposing them. He credits the Podestà of Constantinople, Giovanni Michiel, with commanding the winning side. Akropolites' numbers have more credibility among modern historians, but even on his account the defeat was decisive: the Nicaean fleet lost nearly half its galleys in a single engagement.

Akropolites' account is pointed about the human dimension of the defeat. Manuel Kontophre, the Nicaean fleet commander, had gone to the emperor before the battle and laid out his assessment clearly — that Nicaean sailors lacked the experience to match Venetian professionals in open naval combat. John III disagreed and removed him, replacing him with a man Akropolites describes as "rather hesitant in matters of war," an Armenian officer named Iophre, also rendered in sources as Geoffrey. It was Iophre who commanded the fleet in its defeat. After the battle confirmed Kontophre's judgment, John III restored him to command. The episode says something about how battles can be lost before they begin — not in the fighting, but in the choices made beforehand about who is listened to and who is not.

The defeat was painful but not decisive for the Nicaean campaign overall. According to the French chronicler Alberic of Trois-Fontaines, the naval battle and the surrounding hostilities were followed in June 1241 by a two-year truce among Vatatzes, the Latin Empire, and the Bulgarian ruler Kaliman I. The empire John III had been building did not collapse. His army continued to operate, Nicaean power continued to grow, and nineteen years after the battle of 1241, the Empire of Nicaea retook Constantinople without a fight — in 1261, when a Nicaean force entered the city and the last Latin emperor fled by boat. The galley crews who drowned or surrendered in 1241 did not live to see it.