Venice came to Euboea on a wave of unfamiliar success. For decades the Republic had been losing ground to the Ottoman Empire, but in the Morean War, begun in 1684, the tide had finally turned. Venetian armies, stiffened with mercenary and allied contingents drawn from across western Europe, had swept through the Peloponnese and even captured Athens, where a Venetian mortar shell had famously shattered the Parthenon in 1687. Riding this momentum, the Republic's aging commander, Doge Francesco Morosini, turned north toward Negroponte, the modern Chalkis, the main Ottoman stronghold in Central Greece. Take it, and the Venetians would command the long island of Euboea and the strait that guarded the approach to the Aegean.

Negroponte was no easy prize. It sat astride the narrow Euripus Strait, the same churning, tide-reversing channel that had frustrated armies since antiquity, and its defenses had been built to hold. Around 6,000 Ottoman soldiers manned the walls, and they meant to fight. Morosini landed a powerful force, some 15,000 troops with another 10,000 in the fleet, and opened the siege in July 1688. From the start it went wrong. The Venetians could not fully cut the city off, and the Ottoman commander Ismail Pasha kept the garrison alive, ferrying men and supplies across the water under the besiegers' noses. Every assault that should have ended the siege instead lengthened it, and the longer the army sat outside the walls, the more dangerous its own camp became.

Disease had always been the silent third army in a Mediterranean siege, and at Negroponte it took command. Sickness swept through the crowded, low-lying Venetian camp, and the death toll mounted past anything the Ottoman defenders had inflicted. Roughly four thousand soldiers died, not in battle but in their bedding, fevered and helpless. Among them, on the fifteenth of September, was Otto Wilhelm Königsmarck, the experienced Swedish general who had led much of the army's hard fighting. These were professional soldiers from Germany, Italy, and beyond, men who had survived assaults and storms, undone by an enemy they could not see or fight. The camp that had arrived in summer strength was hollowing out week by week.

On the twelfth of October, the Venetians threw themselves at the walls in a last great assault. It failed, and failed bloodily, and with it failed the whole campaign. The allied force, already gutted by disease, now began to dissolve on its own. The Florentine and Maltese contingents departed. The German mercenaries, looking at the coming winter, flatly refused to spend it camped in the marshes of a fortress they could not take. Morosini, the conqueror of the Morea, had run out of soldiers willing to die for Negroponte. He lifted the siege and withdrew his ruined army back to the Peloponnese, leaving the Ottoman flag still flying over the strait.

Negroponte marked the high-water line of the Venetian advance. The Republic would hold the Peloponnese for another generation, but it never took Euboea, and the failure at Chalkis announced that Ottoman power in Greece was far from broken. Morosini was elected Doge of Venice while still in the field and remains one of the Republic's celebrated commanders, yet the campaign that should have crowned his career instead exposed how little glory and skill could do against fever and dysentery. The fortress on the strait outlasted them all. It would remain in Ottoman hands until Greek independence well over a century later, a monument less to who won the siege than to what had killed the men who lost it.