Spain's hold on the Low Countries had been slipping for half a century, and the Infanta Isabella Clara Eugenia knew the arithmetic. If she could seize the islands of Goeree and Overflakkee, Spanish guns would command the Volkerak strait, choke the Dutch naval base at Hellevoetsluis, and cleave the rebel provinces in half. The propaganda prize sat right there on the mainland: Willemstad, the fortress town named for William the Silent himself, founder of the Dutch revolt. To plant the king's banner over William's town would be answer enough for a decade of humiliations. The plan called for stealth - a fleet of barges, not ships of the line, packed with infantry and slipped quietly out of Antwerp under cover of an autumn night. Command was given on paper to Don Francisco de Moncada, Marquis of Aytona. In practice, it was Count Jan van Nassau-Siegen who led them - a Catholic cousin of the Calvinist House of Orange, fighting his own family for a king in Madrid.

Secrets do not keep in the Scheldt. Word reached the Dutch before the barges cleared Antwerp's harbor, and Vice-Admiral Marinus Hollare moved his Zealandic squadron into the Eastern Scheldt to block the route north. Finding the channel sealed, the Spanish swung south for the island of Tholen instead - somewhere, anywhere, to put boots ashore and claim a victory. They had not reckoned with Colonel Thomas Morgan. The Welsh-born commander led two thousand English and Scots mercenaries garrisoned at Steenbergen, and when he saw what was unfolding, he marched them straight across the seabed. At low tide the shallow flats around Tholen open up into a road for anyone bold enough to take it. Morgan's regiment waded through the salt mud and arrived on the island just as the Spanish boats nosed in. There would be no landing. Van Nassau, his options narrowing by the hour, made a desperate calculation: try the original plan, but in the dark, in fog, through a fleet that was already waiting.

Hollare's nerve held. He watched ninety barges glide past in single file, men crouching to keep their silhouettes off the water, and he did not give the order. Not yet. He waited until the Spanish were fully committed inside the Slaak channel - that narrow ribbon of water between Tholen and Sint-Philipsland where the tide ran fast and the banks closed in - and only then did the Dutch turn into the wakes behind them. There was nowhere to maneuver. The barges were trapped between the Dutch flyboats astern and the shoals along the shore. Hundreds of soldiers leaped overboard in their armor and drowned in chest-deep water. Those who reached the banks found English and Scots mercenaries waiting in the reeds. By dawn on the 13th, more than four thousand men had been captured along with most of the fleet. Van Nassau himself, with only two ships, slipped back toward Antwerp. Perhaps a third of his fleet escaped with him; the rest were Dutch prisoners or dead in the channel.

What happened next reveals something about the Republic at its best. The Admiralty of Amsterdam, following standard practice, recommended throwing every Spanish prisoner into the sea - the official Dutch policy for enemy combatants captured at sea, intended to deter future attempts. Stadtholder Frederick Henry of Orange refused. The prisoners would live. Whether the order was moral, political, or both, it stuck. Among the Dutch captains who distinguished themselves that night was a young Johan Evertsen, who would rise to Lieutenant-Admiral, along with his brother Cornelis Evertsen the Elder, who would do the same. The Slaak slotted into a larger pattern of Habsburg setbacks across the Thirty Years' War - enough that Madrid opened peace talks with the Republic in 1632. The talks failed. The war ground on for another sixteen years. But the islands of Zeeland stayed Dutch, Willemstad kept its name, and Spain never again tried to split the Republic in two.