 On 26 July what begun as an unauthorized attack on the enemy lines by some French knights developed into a major battle as the Duke of Burgundy decided to sally forth in strength.During the Battle of Saint-Omer the Count of Armagnac with his retinue of 300 heavy cavalry succeeded in breaking the Flemish left and then in the following pursuit causing several thousand casualties.Faced with the loss of the bulk of his army Robert of Artois was forced to retreat from the city in disorder.