' History of the Britons' Summary

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. 

The reference in the Historia Brittonum of Arthur carrying the image of St. Mary on his shoulders during a battle has been interpreted by later commentators as a mistake for Arthur bearing the image of Mary on his shield, the error being caused by the similarity between the words in Welsh. 

The greatest classicist of the 19th century, Theodor Mommsen, divided the work into seven parts: Preface (Prefatio Nennii Britonum); I. The Six Ages of the World (de sex aetatibus mundi) (§1-6); II. History of the Britons (historia Brittonum) (§7-49); III. Life of Patrick (vita Patricii) (§50-55); IV. Arthuriana (§ 56); V. Genealogies (regum genealogiae cum computo) (§c. 57—66); VI. Cities of Britain (civitates Britanniae) (§66a); VII. Wonders of Britain (de mirabilibus Britanniae) (§67—76). 

The Historia Brittonum can be dated to about 829. The work was written no earlier than the "fourth year of [the reign of] king Mermenus" (who has been identified as Merfyn Frych ap Gwriad, king of Gwynedd). Historians have conservatively assigned 828 to the earliest date for the work, which is consistent with the statement in chapter 4 that "from the Passion of Christ 796 years have passed. But from his Incarnation are 831 years". 

The text makes use of two narrative techniques which are generally considered not reliable by modern academic standards: synthesizing and synchronizing history. Synthetic history combines legendary elements with fact, which makes the veracity of the text challenging to evaluate. Various specious causal connections and attempts to synchronize material from different sources and traditions also contribute to undermining the reliability of the chronicle.