The Historia Brittonum describes the supposed settlement of Britain by Trojan expatriates and states that Britain took its name after Brutus, a descendant of Aeneas. The work was the “single most important source used by Geoffrey of Monmouth in creating his Historia Regum Britanniae” and via the enormous popularity of the latter work, this version of the earlier history of Britain, including the Trojan origin tradition, would be incorporated into subsequent chronicles for the long-running history of the land, for example the Middle English Brut of England, also known as The Chronicles of England.
The work was the first source to portray King Arthur, who is described as a dux bellorum (‘military leader’) or miles (‘warrior, soldier’) and not as a king. It names the twelve battles that Arthur fought, but unlike the Annales Cambriae, none are assigned actual dates.
Source: Historia Brittonum – Wikipedia