What do you mean? Brittany was more or less 'conquered' by Charlemagne.

Instead of pointlessly warring with the powerful Carolingian Empire, Brittany surrendered their independence and acknowledged Frankish/Carolingian superiority in exchange for being granted tributary status (as opposed to being completely conquered). The Bretons basically acted as semi-autonomous marcher lords. Marcher lords, as opposed to the Counts within the Empire, were allowed more military autonomy to protect their territory - which usually formed the borders of the Empire and were thus often under threat of attack.

This subservience of the Bretons did not last long, though. After Charlemagne's passing, they sensed weakness in the Empire and secured their independence at the Battle of Jengland by defeating the Frankish army led by Charles the Bald. The Bretons would capitalize on this victory, and expand Brittany's territory over the years (at Frankish expense) to reach Brittany's greatest extent in 867.

Sources:

• Einhard's Vita Karoli Magni (817~833).

• Julia M.H. Smith's Province and Empire: Brittany and the Carolingians (1992).