Flanders in the 1380s was the industrial heart of northern Europe. Its cities - Ghent, Bruges, Ypres - made cloth on a scale unmatched anywhere west of the Mediterranean, and their wealth depended on English wool. When Count Louis of Male of Flanders sided with France, the cities sided with England; their looms could not work without English fleeces. Ghent rebelled against Louis in September 1379. After the Flemish rebel army was crushed at the Battle of Roosebeke in November 1382, the citizens of Ghent looked across the Channel for help. London was already at war with France. The Pope was already at war with the Avignon papacy. A military expedition that wore crusading clothing could solve several political problems at once. Henry le Despenser, a bishop with military experience from his youth and an unusually combative theology, was the man Parliament chose to lead it.

The citizens of Ypres had time to prepare. They had watched the English take Gravelines, Dunkirk, Poperinge, and Nieuwpoort along the coast. They had heard about the Franco-Flemish defeat near Dunkirk on 25 May 1383. Under the command of their castellan, John d'Oultre, they organised their defence with the practical seriousness of people who had everything to lose. The outlying suburbs were demolished; the timber was used to strengthen the earth ramparts and the stone gates. A mission was dispatched to Paris to bring back gunpowder. The city was divided into defensive sectors. The ramparts themselves were low - Ypres had been more concerned with trade than war - but they were protected by a double wet ditch, a high thorny hedge reinforced with stakes, and a wooden stockade with a fire-step. When the English and their Ghent allies attacked the Temple Gate on 8 June 1383, the citizens were waiting.

Despenser's army threw itself at the gates for three days. Beaten off. A week into the siege, reinforcements arrived to complete the encirclement; the outer ditch was breached by men shovelling earth into it. On the eighth day, 15 June, the English brought up artillery - a still-novel weapon in 1383, mostly small bombards firing stone balls - and hammered the Messines Gate. They damaged it. They did not breach it. Over the following weeks the bombardment continued without effect; assault after assault was driven off the walls. An attempt to drain the wet ditches got close enough to alarm the defenders before failing. The Ypres garrison managed to slip a message through the lines to the Duke of Burgundy; through him, the city pleaded with Louis of Male, who began raising a large French army to lift the siege. By early August, with disease working through his camp and the French relief force approaching, Despenser's situation was deteriorating fast. On 8 August - exactly nine weeks after the first assault - he abandoned the siege and pulled back. His Ghent allies hung on a little longer; they gave up on 10 September.

The citizens of Ypres did not credit their walls, their gunpowder shipments from Paris, or their castellan with the deliverance of the city. They credited Our Lady of the Enclosure - Onze-Lieve-Vrouw van Tuine, the Virgin Mary in her guise as the patroness of the city. An annual procession in her honour, held on the first Sunday in August, was instituted to commemorate the siege's failure and has continued ever since. It runs today: a costumed Tuindag procession through the streets of Ieper, with banners and figures from the city's medieval history. The English siege of 1383 left other marks too. Ypres never really recovered. The hinterland had been wrecked, the trade with England seriously compromised, and the city's population began a long decline from around 20,000 in 1383 to only 7,600 by the end of the fifteenth century. The looms that had made Ypres rich gradually moved elsewhere.

Despenser's commanders, demoralised and disease-ridden, fell back through Gravelines, took bribes from the approaching French army to abandon the town, and shipped what was left of the crusade home by the end of October. The bishop returned to England in disgrace. He was impeached by Parliament for the failure - a rare proceeding against a bishop - and stripped of his temporalities, though he was eventually restored to his see and continued as Bishop of Norwich until his death in 1406. Ghent fought on alone, until the Peace of Tournai in 1385 with Philip the Bold, the new Duke of Burgundy and Louis of Male's son-in-law and successor. Despenser's Crusade, as it came to be called, is remembered now mostly as a curiosity - the moment when a pope's quarrel and a king's war were spliced together into a single absurd expedition, sold to English congregations as a path to heaven, and ended at the walls of a Flemish cloth town that refused to fall.