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, Theodore 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 oldest surviving manuscript (Harley 3859), which dates from around 1100 and was used for editions of the text by Stevenson, Theodore Mommsen and John Morris, lacks Nennius's prologue. The exemplar for the "Nennian recension" is the one in Cambridge public library ms. Ff. I.27 (base text for Petrie's edition). Gunn used a Vatican palace ms. 1964, which is a copy that ascribes the work to Mark the anchorite. Giles's edition is based on Gunn's, but is a composite since the Vatican ms. (Mommsen's M text) does not contain Nennius's Apologia or the Mirablilia. The anomalous text, the ms. Chartres, which was even older than Harley, was destroyed in World War II, and it ascribed authorship to filius Urbagen (son of Urien). There are also several "Gildasian recensions" (e.g. Cotton Caligula A. VIII), but no one now seriously advocates this document to be penned by Gildas, the near contemporary of Arthur.
Professor David Dumville, enquiring into the stemmatics of the recensions (he has published the Vatican version, Dumville 1985), has branded the Nennian preface (Prefatio Nennii) a late forgery, and believes that the work underwent several anonymous revisions before reaching the forms that now survive in the various families of manuscripts. Dumville's view is largely accepted by current scholarship, though not without its detractors. For a past work that argued for Nennius's authenticity, see Liebermann 1925.
Various introductory notes to this work invoke Nennius's (or the anonymous compiler's) words from the Prefatio that "I heaped together (coacervavi) all I could find" from various sources, not only concrete works in writing but "our ancient traditions" (i.e. oral sources) as well. This is quoted from the Apologia version of the preface. Giles's translation rendered this as "I put together", obscuring the fact that this is indeed a quote from the work and not from some commentator (See Morris's more recent translation as given in wikiquote: Historia Brittonum). Leslie Alcock was not the first to draw attention to the phrase though he may have started the recent spate of interest. However the author still clearly aimed to produce a synchronizing chronicle.
The Historia Brittonum has drawn attention because of its role in influencing the legends and myths surrounding King Arthur. It is the earliest source that presents Arthur as a historical figure, and is the source of several stories which were repeated and amplified by later authors.
The Historia contains a story of the king Vortigern, who allowed the Saxons to settle in the island of Britain in return for the hand of Hengist's daughter. One legend recorded of Vortigern concerns his attempt to build a stronghold near Snowdon, called Dinas Emrys, only to have his building materials disappear each time he tries. His advisers tell him to sprinkle the blood of a boy born without a father on the site to lift the curse. Vortigern finds such a youth in Ambrosius, who rebukes the wise men and reveals that the cause of the disturbance is two serpents buried under the ground.
The tower story is repeated and embellished by Geoffrey of Monmouth in his Historia Regum Britanniae, though he attributes it to Merlin, saying "Ambrosius" is the sage's alternative name. Geoffrey also includes Aurelius Ambrosius, another figure mentioned in the Historia, as a king in his own right, and also includes other characters such as Vortimer and Bishop Germanus of Auxerre.
Chapter 56 discusses twelve battles fought and won by Arthur, here called dux bellorum (war leader) rather than king:
At that time, the Saxons grew strong by virtue of their large number and increased in power in Britain. Hengist having died, however, his son Octha crossed from the northern part of Britain to the kingdom of Kent and from him are descended the kings of Kent. Then Arthur along with the kings of Britain fought against them in those days, but Arthur himself was the military commander ["dux bellorum"]. His first battle was at the mouth of the river which is called Glein. His second, third, fourth, and fifth battles were above another river which is called Dubglas and is in the region of Linnuis. The sixth battle was above the river which is called Bassas. The seventh battle was in the forest of Celidon, that is Cat Coit Celidon. The eighth battle was at the fortress of Guinnion, in which Arthur carried the image of holy Mary ever virgin on his shoulders; and the pagans were put to flight on that day. And through the power of our Lord Jesus Christ and through the power of the blessed Virgin Mary his mother there was great slaughter among them. The ninth battle was waged in the City of the Legion. The tenth battle was waged on the banks of a river which is called Tribruit. The eleventh battle was fought on the mountain which is called Agnet. The twelfth battle was on Mount Badon in which there fell in one day 960 men from one charge by Arthur; and no one struck them down except Arthur himself, and in all the wars he emerged as victor. And while they were being defeated in all the battles, they were seeking assistance from Germany and their numbers were being augmented many times over without interruption. And they brought over kings from Germany that they might reign over them in Britain, right down to the time in which Ida reigned, who was son of Eobba. He was the first king in Bernicia, i.e., in Berneich.
Most of these battle sites are obscure and cannot be identified. Some of the battles appear in other Welsh literature, though not all are connected explicitly with Arthur. Some scholars have proposed that the author took the list from a now-lost Old Welsh poem which listed Arthur's twelve great victories, based on the fact that some of the names appear to rhyme and the suggestion that the odd description of Arthur bearing the image of the Virgin Mary on his shoulders at Guinnion might contain a confusion of the Welsh word iscuit (shield) for iscuid (shoulders). Others reject this as untenable, arguing instead that the author included battles which were not previously associated with Arthur or perhaps made them up entirely.
A similar story to that attached to Guinnion also appears in the Annales Cambriae; here, Arthur is described as carrying "the cross of our Lord Jesus Christ on his shoulders for three days and three nights…", though here the battle is said to be Badon rather than Guinnon. T. M. Charles-Edwards argues that these accounts both refer to a single source. Other scholars, however, such as Thomas Jones and N. J. Higham, argue that the Annales account is based directly on the Historia, suggesting the name of the battle was switched from the unknown Guinnon to the famous Badon, and that the icon Arthur carries was replaced with a more common one.
The Battle of Mount Badon is associated with Arthur in several later texts, but not in any that predate the Historia. It was clearly a historical battle, being described by Gildas, who does not mention the name of the Britons' leader (he does, however, mention Aurelius Ambrosius as a great scourge of the Saxons immediately prior.) Of the other battles, only the Battle of Tribuit is generally agreed to be associated with Arthur in another early Welsh source. Tribuit appears as Tryfrwyd in the Old Welsh poem Pa Gur?, dating to perhaps the mid-ninth century. In this poem, it follows the story of a battle against cinbin, or dogheads, whom Arthur's men fight in the mountains of Eidyn (Edinburgh); in the Tryfrwyd battle they spar with a character named Garwlwyd (Rough-Gray), who is likely identical with the Gwrgi Garwlwyd (Man-Dog Rough-Grey) who appears in one of the Welsh Triads. Arthur's main protagonist in the fight is Bedwyr, later known as Sir Bedivere, and the poem also mentions the euhemerized god Manawydan. "The City of the Legion" may be a reference to Caerleon, whose name translates as such, but it might also refer to Chester, the site of a large Roman base.
Cat Coit Celidon is probably a reference to the Caledonian Forest (Coed Celyddon) which once covered the Southern Uplands of Scotland. Scholar Marged Haycock has suggested this battle can be identified with the Cad Goddeu, the "Battle of the Trees", best known from the tenth-century poem Cad Goddeu. Arthur is mentioned towards the end of this poem, and a fragment of a story about the battle preserved in manuscript Peniarth 98B states that the battle had an alternate name, Cad Achren, which suggests a connection with the Caer Ochren raided by Arthur in the earlier poem Preiddeu Annwfn.
Various writers have asserted that this chapter supports a historical basis for King Arthur and have tried to identify the twelve battles with historical feuds or locales (see Sites and places associated with Arthurian legend). On the other hand, Thomas Green argues that the only identifiable battles linked explicitly with Arthur in Old Welsh sources are mythological, undermining any claims that the battles had a basis in history.
Attached to the Historia is a section called De mirabilibus britanniae (or simply Mirabilia in shorhand). It gives a list of 13 topological marvels, or wonders of Britain, followed by a few marvels of Anglesey (Menand insulae or Mona) and of Ireland.
The Mirabilia section is thought to be not part of the original work, but is believed to have been composed shortly after, and is attached to many, though not all of the manuscripts.
Two of the marvels are Arthurian lore (Chapter 73 of the Historia). It might be worth noting that old editions give "Troynt" as the name of the great boar and "Anir" as the name of Arthur's tragic son, from the Harleian manuscript, but Fletcher suggested the variant readings "Troit" and "Amr" be preferred (since they are closer to the Welsh forms of those names).
The first concerns Arthur's dog, Cabal (Cavall in Welsh) and the footprint it left while chasing the boar Troynt (→Troit) Twrch Trwyth:
There is another marvel in the region which is called Buelt. There is a mound of stones there and one stone placed above the pile with the pawprint of a dog in it. When Cabal, who was the dog of Arthur the soldier, was hunting the boar Troynt, he impressed his print in the stone, and afterwards Arthur assembled a stone mound under the stone with the print of his dog, and it is called the Carn Cabal. And men come and remove the stone in their hands for the length of a day and a night; and on the next day it is found on top of its mound.
The second concerns Arthur's son Anir or Amr (Amhar in Welsh) and his sepulcher:
There is another wonder in the region which is called Ercing. A tomb is located there next to a spring which is called Licat Amr; and the name of the man who is buried in the tomb was called thus: Amr(←Anir). He was the son of Arthur the soldier, and Arthur himself killed and buried him in that very place. And men come to measure the grave and find it sometimes six feet in length, sometimes nine, sometimes twelve, sometimes fifteen. At whatever length you might measure it at one time, a second time you will not find it to have the same length—and I myself have put this to the test.
There are also chapters relating events about Saint Germanus of Auxerre that claim to be excerpts from a (now lost) biography about this saint, a unique collection of traditions about Saint Patrick, as well as a section describing events in the North of England in the sixth and seventh centuries which begins with a paragraph about the beginnings of Welsh literature (ch. 62):
At that time, Talhaiarn Cataguen was famed for poetry, and Neirin, and Taliesin and Bluchbard, and Cian, who is called Guenith Guaut, were all famous at the same time in British poetry.
There are a number of works that are frequently associated with the Historia Brittonum, in part because some of them first appear with the text preserved in the Harleian manuscript, and partly because whenever the Historia Britonum is studied, these sources eventually are mentioned.The Lebor Bretnach. An Irish translation of the Historia Brittonum (Todd 1848 ed. tr.), and a recension of the 'Nennian' Historia Brittonum.
The Annales Cambriae. This is a chronicle consisting of a series of unnumbered years, from AD 445 to 977, some of which have events added. Two notable events are next to AD 516, which describes The Battle of Badon, and 537, which describes the Battle of Camlann, "in which Arthur and Mordred fell." A version of this was used as a starting point for later Welsh Chronicles.
Welsh Genealogies. One of many collections of Welsh genealogies, this documents the lineage of Hywel Dda, king of Wales, and several of his contemporaries. The Pillar of Eliseg is frequently discussed in connection with these genealogies.
Anglo-Saxon Genealogies. This is a version of a collection of the genealogies of five pre-Viking kingdoms -- Bernicia, Deira, Kent, East Anglia, and Mercia—better known in its independent form.