The siege had a year's worth of grievance behind it. In 1419 Burgundy had allied with England against France in the long war over the French crown. That alliance held until 1435, when the powers met at the Congress of Arras to negotiate a peace. The English walked out. The Burgundians did not. On 21 September 1435, Philip the Good signed the Treaty of Arras with Charles VII of France and switched sides. In London the news was received as treachery, and Londoners were given leave to plunder the houses of Flemish, Dutch and Picard merchants in the city - subjects of a duke who, the previous summer, had been their ally. An English raiding force of two thousand men crossed the Channel and defeated fifteen hundred Flemish soldiers under Jean II de Croy in the Boulonnais. Philip declared war. The Flemish cities cheered him on. Their target, of course, was Calais.

The Duke moved methodically. He took the smaller English outposts first - Oye castle, where he hanged part of the garrison, then Sangatte and Balinghem - and detached Jean II de Croy with a force to besiege Guines while the main army surrounded Calais in June. Thirty thousand men, on paper, is a serious number. But the Calais garrison under Edmund Beaufort, Count of Mortain, was well provisioned and well led, and the city's defences had been improved continuously since the English had taken it in 1347. The walls did not fall in the first week. They did not fall in the second. The Flemish admirers of the campaign had imagined a quick civic triumph; what they got was a long, expensive, professional siege.

The plan was to seal the port. Admiral John of Horne brought a fleet to the harbour mouth and loaded five or six ships with stone, intending to scuttle them as blockships and choke off English resupply. The plan failed. The blockships did not seal the channel. English ships kept coming in - food, arrows, fresh garrison, the steady drip of reinforcement that an island power could pour into a Channel port at will. Once the harbour stayed open, the siege was effectively over. The militia knew it before the duke did. The Flemish contingents, who had marched out as enthusiastic civic militias, began to disintegrate. Men slipped away in twos and tens, then in companies. Discipline collapsed in the lines.

Across the Channel an English relief army of around ten thousand men was assembling under Humphrey, Duke of Gloucester, the king's uncle and Lord Protector. When Gloucester's force began to cross, Philip the Good had to make a quick decision. His army was already coming apart; meeting a fresh ten thousand professional English troops with a dwindling Flemish militia was not a battle plan. He raised the sieges of Calais and Guines and withdrew. Among the dead left behind was Guy of Burgundy, Lord of Kruibeke - Philip's own half-brother, an illegitimate son of John the Fearless and Marguerite de Borsele. The duke went home humiliated.

The political collapse of the siege was followed by years of low-grade attrition. English raids and English piracy hammered Flemish trade. But the English economy, too, depended on the wool that moved through Calais to Flemish looms. By 1439 both sides had bled enough. Peace returned, the Calais Staple resumed its work, and the city stayed English for another 122 years - the longest-running English possession in mainland France. It would finally fall to the French in 1558, not to the Burgundians whose great-grandchildren were by then Habsburgs. The siege of 1436 is remembered, if it is remembered at all, as the moment when civic enthusiasm met the cold arithmetic of professional war and lost. Calais had walls. The militia had jobs to get back to. The walls won.