The islands of Zeeland were a contested inheritance, fought over for three centuries by the counts of Flanders to the south and the counts of Holland to the north. In 1303 the Flemish, fresh from their stunning infantry victory at the Battle of the Golden Spurs, decided to settle the question. Guy of Namur sailed a fleet out of Sluis on 23 April and landed near Arnemuiden and Veere. Middelburg surrendered. The whole island of Walcheren fell. Only the port of Zierikzee, perched on the north shore of Schouwen, held out for the Count of Holland. The Flemings besieged it. The siege dragged on for over a year. By summer 1304 the city was starving - and the conflict had grown much larger than Zeeland. Flemish armies were now marching on Saint-Omer and Tournai, threatening King Philip IV of France himself. So Philip did what medieval kings did when they needed competent sea power: he hired Italians.

Rainier Grimaldi was already a famous corsair, the kind of Mediterranean operator whose family motto might as well have been *opportunity*. The Grimaldis had been agitating for control of the rock of Monaco for decades. Now Rainier sailed his eleven galleys north through the Atlantic, picked up thirty French cogs and eight Spanish cogs at the channel, met the small Holland squadron at Schiedam, and turned the combined force toward Zierikzee. Against him: a Flemish fleet of thirty-seven ships drawn from across northern Europe - English, Hanseatic, Spanish, Swedish, Flemish - plus a swarm of smaller boats. On the evening of 10 August 1304, the two fleets met on the Gouwe, a shallow tidal bay that no longer exists, since silted into farmland. The cogs - tall, deep-drafted vessels built for the open North Sea - immediately ran into trouble. The Flemish smaller boats danced around them in the silt. A Flemish fire ship attack was launched. The wind blew the fire ships back into the Flemish line.

The first day's fighting went badly for Grimaldi. His larger French ships grounded on the mud as the tide ebbed, and the Flemings - with shallower drafts - circled and harried them. The Genoese galleys, designed for the deep Mediterranean, were not built for this. But Rainier was patient, and tides come back. As the water rose through the night, the grounded cogs floated free. By the time dawn broke on 11 August, Grimaldi's heavy ships were back in the fight, and the balance had shifted. What happened next is one of those stories that medieval chroniclers loved and modern historians cannot quite verify: someone in the Flemish fleet, a traitor or a saboteur, cut the moorings of his own ships. When morning came, the Flemish vessels were drifting helplessly through the channel, no longer in formation, no longer able to fight as a unit. Grimaldi's fleet was still in battle array. The slaughter was efficient. Guy of Namur was taken alive. The siege of Zierikzee was lifted.

One week later, on 18 August 1304, Philip IV himself defeated the main Flemish army at Mons-en-Pévèle. The Flemish dream of holding Zeeland was finished. Within a generation the Eastern Scheldt would silt up further, the Gouwe would close, and Schouwen-Duiveland would consolidate into something close to its modern shape. Zierikzee kept its medieval city gates - the Noordhavenpoort and the Nobelpoort still stand today - and prospered through the fourteenth century as one of the great ports of the southern North Sea. Rainier Grimaldi sailed home with his reputation made, became Admiral of France, and used the prestige and the prize money to advance his family's long campaign for Monaco. His descendants got there eventually. The Battle of Zierikzee is the unlikeliest line on the Grimaldi family tree - the day the founder of Monaco's dynasty saved a Dutch town from a Flemish siege by waiting for the tide.