The Dutch Revolt - what became the Eighty Years' War - had been going for six years by 1574. The provinces of the Low Countries had risen against their Habsburg overlord, Philip II of Spain, and against the harsh rule of his governor-general, the Duke of Alba. Holland and Zeeland had largely thrown the Spanish out by 1572. Only Amsterdam remained loyal to Madrid. Alba responded with terror campaigns intended to break rebel morale: when his soldiers took Naarden, they killed almost everyone in the town. When they took Haarlem after a seven-month siege, the massacres continued. The pattern was understood on both sides. Surrender meant slaughter. Resistance meant siege. In October 1573 the Spanish commander Francisco de Valdez surrounded Leiden, in southern Holland, and prepared to do it again.

Leiden had food when the siege began. The defenders - English, Scottish, and Huguenot French volunteers alongside the city's own men - held the walls and waited for relief. William of Orange, the leader of the Dutch rebels, sent his brother Louis of Nassau with an army to break the siege. Valdez lifted the siege in April 1574 to march out and meet Louis - but the Spanish reached him first. At the Battle of Mookerheyde, Louis of Nassau was killed and his army shattered. By the time Valdez returned to Leiden in May 1574, the city had failed to use the respite to restock. The second siege began on a city already weakening. Through the summer of 1574, supplies dwindled. The plague arrived. Thousands began to die. The mayor, Pieter van der Werff, told his starving citizens that if they wanted to surrender, they would have to kill him first, and they could eat his arm if they were really that desperate.

William of Orange's plan was one of the most audacious military operations of the century. He could not march an army through Spanish-held countryside to relieve Leiden. So he proposed, instead, to let the sea in. On 3 August 1574, the outer dykes were broken. The intention was to flood the polders between the coast and Leiden, then sail the rebel fleet across the new shallow sea to the besieged city. The cost was enormous - the breach destroyed years of agriculture in some of the most productive farmland in Europe, and the surrounding peasants resisted the order until William prevailed. Then nothing happened. The winds turned easterly and pushed the water back out to sea. The flooding spread too slowly. William, ill with fever, lay in bed in Rotterdam while the fleet waited. Inside Leiden, the inhabitants ran out of food. By 21 August they wrote that they had endured three months of siege - two with food, one without. William replied by carrier pigeon: the dykes were broken, the relief was coming, hold on.

On 1 September the rebel fleet under Admiral Louis de Boisot - more than 200 flat-bottomed vessels with 2,500 hardened sailors and provisions for the starving - began to move. They captured the Landscheiding dyke on the night of 10 September in a surprise attack. They breached the Greenway dike a mile further on. But the flooding was too shallow for the boats to cross the open polders, and the only navigable channel was a canal heavily defended by the Spanish. Boisot pulled back and sent a despairing message: unless the wind changed, all was lost. On 18 September the wind shifted hard from the west and piled the North Sea against the Dutch coast. The water rose. The fleet got moving. By the end of September the Spanish strongholds in the flooded countryside had been abandoned one by one, the soldiers panicking at the inexorable advance of seawater. Only the fort at Lammen, under Colonel Borgia, still stood between Boisot and Leiden. Then the wall collapsed inside the city, the Spanish at Lammen mistook the noise for another deliberate breach, and they retreated in the night. The relief sailed in on 3 October.

The first food into the city was herring and white bread - simple, dense, exactly what starving people could keep down. By evening they had hot stew. The legend, repeated ever since, has it that an orphan boy named Cornelis Joppenszoon climbed into the abandoned Spanish camp at Lammenschans and found a still-warm pot of hutspot - a stew of carrots, onions, and parsnips that the Spaniards had cooked just before fleeing. To this day, on the third of October, Leiden celebrates Leidens Ontzet - the Relief of Leiden. The municipality hands out free herring and white bread to citizens, families eat hutspot for dinner, there is a funfair, and the streets fill with people. It is one of the most genuinely felt civic festivals in the Netherlands. As his thanks to the city for what it had endured, William of Orange founded Leiden University in February 1575. He framed the founding charter as the act of the king he was in fact rebelling against - a small piece of legal fiction that no one believed and everyone respected. Leiden University is the oldest in the Netherlands. Within decades it had become one of the great universities of Europe, and it still anchors the city today, in the same canal-laced streets that once watched the rising flood and waited for the boats.