By the mid-14th century, the Genoese merchant colony of Galata—just across the Golden Horn from Constantinople—had effectively become more powerful than the empire that tolerated it. A Byzantine historian named Nikephoros Gregoras recorded the numbers that made the relationship humiliating: in 1348, the customs revenues collected at Galata were 200,000 in standard currency, while those collected by Constantinople itself amounted to just 30,000. The Genoese had turned a commercial concession granted by the Treaty of Nymphaeum in 1261 into a stranglehold on Byzantine trade. When Emperor John VI Kantakouzenos tried to push back by rebuilding the imperial fleet, the Genoese captured those new ships and blockaded his own capital. They also controlled access through the Bosporus to the Black Sea, threatening to shut Venice out of the region's lucrative trade entirely. Venice responded by making alliances: with Byzantium in 1351, and with the Crown of Aragon, which agreed to contribute a fleet.

The Genoese commander Paganino Doria had sailed from Genoa in July 1351 with sixty ships, each carrying 180 men. He was there to defend Galata and the Genoese position. The allied response under the Venetian admiral Niccolò Pisani was slower and plagued by setbacks—storms delayed the fleet, ships were lost en route, and Doria managed to blockade Pisani at the Venetian colony of Negroponte (modern Chalkis, in Greece) until an Aragonese fleet of forty galleys under Ponç de Santa Pau arrived to break the standoff. By early February 1352, Pisani's refloated squadron and the Aragonese had finally joined forces near the Princes' Islands, southeast of the Bosporus entrance. The allied fleet numbered 89 or 90 ships: 45 Venetian, 30 Aragonese, and 14 Byzantine. The Genoese had 60 to 64. By sheer numbers, the battle should not have been close.

Doria tried to engage the Aragonese and Venetians before they could link up with the Byzantines, but contrary winds stopped him. He withdrew north into the Bosporus, and the allied fleet followed. The two forces met in the narrow waters near Diplokionion—the district now known as Beşiktaş, on the European shore of Istanbul. The Byzantine ships, despite being part of the alliance that had brought this battle about, apparently never engaged; they fled before the fighting began. The storm intensified as night fell. Both sides tried to form battle lines and could not. In the confusion of darkness, crashing waves, and tangled rigging, sailors on opposing ships could not tell friend from enemy. Genoese galleys lost 16 ships. The Venetians and Aragonese lost 23, including admiral Pancrazio Giustinian. The Aragonese, fighting in waters they did not know, suffered disproportionately. When the allies finally withdrew, the Genoese—who had held the field, though barely—were the victors by the only measure that counted: they were still there.

The aftermath revealed how unclear victory had been. Franciscan friars attempted to arrange a prisoner exchange and found almost no one to exchange—the sea and the storm had taken most of the missing. Aragón's commander, Santapau, wrote to King Peter IV claiming to have captured 23 Genoese galleys. In reality, the battle was what it looked like: an atrocious draw fought in terrible conditions, claimed by both sides and decisive for neither. But strategically, the Genoese position held. Abandoned by their allies, the Byzantines had no choice but to settle. In a treaty signed on May 6, 1352, Emperor Kantakouzenos formally recognized Genoese possession of Galata, barred Venetian and Aragonese ships from his ports, and agreed to restrict his own merchants' trade without Genoese consent. The colony had won. Doria returned to Genoa in August—but the losses were so severe he received no honors for the campaign. The men who drowned in the Bosporus that February night were buried by the current, in the same cold water that had always divided Europe from Asia.