Three weeks after their arrival at Rouen the Bretons broke out in insurrection, and the duke invited Harold to accompany him on an expedition to subdue them, courteously saying that he should obtain great advantage from the military experience of his guest.

Harold accepted the invitation, and with his thanes set out with the duke. Against the disciplined forces of Normandy the Breton peasants had no chance whatever in the open field, but their wild and broken country, well-nigh covered with forest, afforded them an opportunity for the display of their own method of fighting by sudden surprises and attacks, and they defended their rough but formidable intrenchments with desperate valour. Harold's experience gained in his warfare with the Welsh was of much utility, and the duke committed to his hands the formal command of the troops engaged, averring that he himself knew comparatively little of warfare such as this.

Harold conducted the operations with equal vigour and prudence. Stronghold after stronghold was attacked and captured, a small portion of the force only being engaged in active operations, whilst the rest were posted so as to repel the attempts of the Bretons to aid the besieged. By his advice clemency was always shown to the garrisons when the fortresses were stormed, and they were permitted to return unharmed to their people, bearing the news that the duke bore no ill-will towards them, and was ready to show mercy to all who laid down their arms. Wulf and Beorn were permitted to share in the assaults, and with the Saxon thanes followed Harold, as he led the way on foot up to the intrenchments at one point, while the duke with a party of his barons attacked at another. More than once the English banner was carried into the heart of the Breton fortress before the Normans had fought their way in, and on each of these occasions the duke warmly expressed his admiration for the courage of his English allies. At last there remained but one formidable stronghold to be captured, and so strong was this by nature, and so desperately defended, that for some time the efforts of the besiegers were fruitless.