On 25 May the crusaders put to flight a Franco-Flemish army, under the command of Louis of Male, in a pitched battle fought near Dunkirk.

Although the ramparts were low, they were well protected with a double wet ditch, a high thorny hedge reinforced with stakes, and a wooden stockade and fire-step.

Before the end of the first week of the siege, reinforcements arrived to completely encircle the city walls and the outer ditch was breached using soil.

[6] On 8 August, after nine weeks of effort, Despenser abruptly decided to abandon the siege, as did his Ghent allies on September 10.

In Ypres, the victory was attributed to the intervention of Our Lady of the Enclosure, in whose honour an annual procession has since been held on the first Sunday in August.