What Beauchamp wanted was a position that turned cavalry numbers against themselves. He found one. A river bend, a wide ditch in front of his line, marshes that constrained the approach. The French would not be able to bring all fifteen hundred men-at-arms to bear at once. He sent the wounded and the plunder ahead toward Calais. The healthy fighters formed up in the river bend. The opening of the battle came as a gift: Edouard I de Beaujeu, the French commander, was killed before the lines had really closed. The men around him kept coming anyway. They had Geoffroi de Charny - the same Charny who had been ransomed back from English captivity after the failed bribe attempt on Calais the year before - sharing command, and they had enough men to absorb the loss of a leader and keep pressing.

An English defensive position in the mid-fourteenth century lived and died by its archers. The longbow had broken the French at Crecy in 1346 and would break them again at Poitiers in 1356; it was the weapon that made small English forces dangerous to large French ones. Beauchamp's three hundred mounted archers shot until they had nothing left to shoot. When the arrows ran out, the archers, instead of breaking, charged on foot. It was a brave gesture and an incoherent one. Archers were not equipped for the kind of close-quarters melee that men-at-arms in plate had trained for since boyhood. The charge had little substantial effect. Then a fresh French cavalry detachment arrived from Saint-Omer and the English line, already battered, gave way.

Beauchamp himself was taken, along with many of his men. The medieval custom of ransom meant that capture of a knight was, in business terms, an investment rather than an execution; the French commanders had reason to want him alive. The infantry on both sides were treated differently - the chronicles, as they almost never do, leave their fate uncounted - but for the knightly classes, capture meant a long negotiation back to freedom. Charny, who knew the process intimately, would have understood every step of what was about to unfold for his English counterpart. The French had lost their nominal commander Beaujeu at the start of the fight; they had gained the English commander at the end. By the strange accounting of fourteenth-century warfare, that was a clear French victory.

The aftermath was the most dangerous moment for the English in northern France. A significant portion of the Calais garrison's mobile strength had just been killed or captured in a single afternoon. The Pale of Calais sat on a narrow ring of fortifications around the city - Guines, Sangatte, Marck, Fretun - and depended on a strong central reserve at Calais to defend any threatened point. That reserve was suddenly much thinner. If the French had been able to push hard on Calais immediately, in the days right after Ardres, they might have done damage that none of the earlier sieges had managed. They did not. The French force that won at Ardres was not organised for a follow-on offensive, and English reinforcements arrived at the garrison before the French could capitalise. The window closed.

The Battle of Ardres did not change the war. It did not even change Calais - the town remained English for another two centuries. But it is one of the rare reminders that English dominance in the early Hundred Years' War was not automatic. Crecy and Poitiers tell a story of longbows and English tactical superiority; Ardres tells a story of running out of arrows in a river bend and being overrun by reinforcements. The English commander Beauchamp would have time, during his captivity, to think about the choice between fighting in the open and fighting from a defensive position. So would Charny, whose career was an unbroken sequence of opportunities to think about exactly that. Three years later, the truce was renewed; five years after that, at Poitiers, Charny died holding the Oriflamme as French resistance collapsed for a generation. Ardres, by then, was a small line in the chronicles. But on a June afternoon in 1351, in a river bend, it was the war.