Geoffrey wrote several works of interest, all in Latin, the language of learning and literature in Europe during the medieval period. His major work was the Historia Regum Britanniae (History of the Kings of Britain), the work best known to modern readers. It relates the purported history of Britain, from its first settlement by Brutus, a descendant of the Trojan hero Aeneas, to the death of Cadwallader in the 7th century, taking in Julius Caesar's invasions of Britain, two kings, Leir and Cymbeline, later immortalized by William Shakespeare, and one of the earliest developed narratives of King Arthur.
Geoffrey claims in his dedication that the book is a translation of an "ancient book in the British language that told in orderly fashion the deeds of all the kings of Britain", given to him by Walter, Archdeacon of Oxford. Modern historians have dismissed this claim. It is, however, likely that the Archdeacon furnished Geoffrey with some materials in the Welsh language that helped inspire his work, as Geoffrey's position and acquaintance with the Archdeacon would not have afforded him the luxury of fabricating such a claim outright. Much of it is based on the Historia Britonum, a 9th century Welsh-Latin historical compilation, Bede's Historia ecclesiastica gentis Anglorum and Gildas's sixth-century polemic De Excidio et Conquestu Britanniae expanded with material from Bardic oral tradition, genealogical tracts, and embellished by Geoffrey's own imagination. In an exchange of manuscript material for their own histories, Robert of Torigny gave Henry of Huntingdon a copy of Historia regum Britanniae, which both Robert and Henry used uncritically as authentic history and subsequently used in their own works, by which means some of Geoffrey's fictions became embedded in popular history.
Historia Regum Britanniae is now acknowledged as a literary work of national myth containing little reliable history. This has since led many modern scholars to agree with William of Newburgh, who wrote around 1190 that "it is quite clear that everything this man wrote about Arthur and his successors, or indeed about his predecessors from Vortigern onwards, was made up, partly by himself and partly by others". Other contemporaries were similarly unconvinced by Geoffrey's "History". For example, Giraldus Cambrensis recounts the experience of a man possessed by demons: "If the evil spirits oppressed him too much, the Gospel of St John was placed on his bosom, when, like birds, they immediately vanished; but when the book was removed, and the History of the Britons by 'Geoffrey Arthur' (as Geoffrey named himself) was substituted in its place, they instantly reappeared in greater numbers, and remained a longer time than usual on his body and on the book."
However, his major work was widely disseminated across the whole of Medieval Western Europe: Acton Griscom listed 186 extant manuscripts in 1929, and others have been identified since. It enjoyed a significant afterlife in a variety of forms, including translations/adaptations such as the Anglo-Norman Roman de Brut of Wace, the Middle English Brut of Layamon, and several anonymous Middle Welsh versions known as Brut y Brenhinedd ("Brut of the kings"). where it was generally accepted as a true account.
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