The reign of Arthur is the section of Wace's work where the poet takes the greatest liberties with his sources. Most notably, it is the episode where the historical stance appears to be at its weakest, with the intrusion of material that is neither didactic nor religious in origin. It is also the fullest account of any of the Celtic kings of Britain, covering two Books (Books IX and X) or 37 chapters (out of a total of 207) in the Latin sources. Wace's account of Arthur's reign is not a homogeneous, uniform whole. It develops in three stages, starting with the young king's struggle to establish his authority over Britain, during which he frees the land from the Saxon threat; continuing with his glorious mature years, which see him conquer Norway and France; and ending with the reappearance of the old enemy, Rome, and Modred's treason.

Young Arthur

Wace takes relatively little interest in the early, defensive exploits of Arthur, the high point of the Arthurian section being the foundation of the Round Table, the conquest of France and the glories of the Whitsun court at Caerleon (at which the Roman ultimatum is delivered). The account of the youthful exploits of the once and future king follows the Variant text quite closely, with only very minor, predictable additions such as the fact that the Saxon Baldulf, who has disguised himself as a jongleur to gain access to his brother through enemy lines, had been taught to sing and play the harp (9103–4). Despite the occasional rhetorical passage (for example, descriptions of Saxon débâcle, 9179–82 and 9395–406), the trend is towards compression and narrative economy. This is effected by the omission of information, albeit on a limited scale, and through the subtle reshaping of certain episodes.

The focus is firmly on Arthur from the outset. The scene where Arthur arms himself before the battle of Bath remains close to the account of the First Variant version, but Wace adds iron foot-protection to the young king's equipment, and mentions that his jewel-encrusted helmet had belonged to Uther (9283–8), thus reinforcing both the scene's realism and its dynastic significance.