Battle of Cassel

On February 22, 1071, on the heights of Cassel, Robert the Frisian faced the army of his nephew Arnoul III supported by Richilde of Hainaut, by King Philip I and by imperial contingents. After several hours of fierce melee, Robert managed to break through the opposing vanguard, took Count Eustache de Boulogne prisoner and killed Arnoul himself, causing the rout of the loyalist party.

On the death of Baldwin VI of Flanders, the regency of his minor son Arnoul III was contested by Robert the Frisian, brother of the deceased, who benefited from the support of the Flemish maritime cities. Richilde seeks the support of the King of France and the Holy Roman Empire, bringing together in Cassel a composite army mixing Hennuyer knights, Picards and royal troops. Robert relies on the urban militia of Bruges, on seasoned Flemish knights and on an excellent knowledge of the hills of Cassel.

Robert divided his forces into three battles and occupied the Cassel Ridge, forcing the enemy to climb the soggy slope. After a first frontal shock, he ordered his flank reserve to surround the Hennuyère column while the urban archers sprayed the enemy below. The topography and cohesion of the Flemish cavalry allowed it to break through the opposing center, isolating Arnoul and his guards who were overwhelmed.

The death of Arnoul and the temporary capture of Richilde put an end to the war of succession: Robert was recognized as Count of Flanders after having negotiated the release of King Philip I, injured and captured in the melee. The new balance consolidates Flemish autonomy while forcing Robert to come to terms with the Capetian royalty, whose ally he becomes against imperial incursions. The Flemish towns gain an interlocutor favorable to their privileges.