De Excidio et Conquestu Britanniae

It is a sermon in three parts condemning the acts of Gildas' contemporaries, both secular and religious, whom he blames for the dire state of affairs in sub-Roman Britain.

Part I contains a narrative of British history from the Roman conquest to Gildas' time; it includes references to Ambrosius Aurelianus and the Britons' victory against the Saxons at the Battle of Mons Badonicus.

Gildas's work is of great importance to historians, because, although it is not intended primarily as history, it is almost the only surviving source written by a near-contemporary of British events in the fifth and sixth centuries.

Andrew Breeze argues that Gildas was writing De Excidio in 536, in the middle of the extreme weather events of 535–536, because he mentions a "certain thick mist and black night" which "sits upon the whole island" of Britain.

[9] The oldest manuscript of the De Excidio is Cottonian MS. Vitellius A. VI, of the tenth century, damaged by fire in 1731, but used by Theodor Mommsen in his edition nevertheless.

The oldest attestation of Gildas's work is actually found in the extensive quotations and paraphrases of the De Excidio made by Bede in his Ecclesiastical History of the English People, the earliest manuscripts of which date to the eighth century.

The text of Gildas founded on Gale's edition collated with two other manuscripts, with elaborate introductions, is included in the Monumenta Historica Britannica.

The text as it is used today is thus a scholarly reconstruction; the prime witness and possibly the entire manuscript stemma may not actually preserve the original page order of the autograph.

John Edward Lloyd suggests a connection between this king and the descendants of the great hero Ambrosius Aurelianus mentioned previously by Gildas; if this is true his kingdom may have been located somewhere in territory subsequently taken by the Anglo-Saxons.

[26] If the form Caninus should be connected with the Cuna(g)nus found in 6th-century writings, the result in the later royal genealogies would be Cynan, a commonly occurring name.

Though it is not easily supportable on linguistic grounds, some scholars maintain that he is mentioned on a memorial stone (discovered in 1895) bearing an inscriptions in both Latin and ogham.

Gildas continues his jeremiad against the clergy of his age but does not explicitly mention any names in this section, and so does not cast any light on the history of the Church in this period.

Following the conquest of Britain described in De excidio, Gildas continued to provide an important model for Anglo-Saxon writers both in Latin and in English.

In the later Old English period, Gildas's writing provides a major model for Alcuin's treatment of the Viking invasions, in particular his letters relating to the sack of Lindisfarne in 793.

Likewise, Wulfstan of York draws on Gildas to make a similar point in his sermons, particularly in the Sermo Lupi ad Anglos.

Gildas uses Latin to address the rulers he excoriates and regards Britons, at least to some degree, as Roman citizens, despite the collapse of central imperial authority.