De excidio Britanniae

De Excidio et Conquestu Britanniae (Latin for "On the Ruin and Conquest of Britain") is a work by the 6th-century British cleric St Gildas. 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. It is one of the most important sources for the history of Britain in the 5th and 6th centuries, as it is the only significant source for the period written by a near contemporary of the people and events described.

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. Part II is a condemnation of five kings for their various sins, including both obscure figures and relatively well-documented ones such as Maelgwn Gwynedd. Part III is a similar attack upon the British clergy of the age.

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. The usual date that has been given for the composition of the work is some time in the 540s, but it is now regarded as quite possibly earlier, in the first quarter of the sixth century, or even before that. Cambridge historian Karen George, in her exhaustive study of Gildas' text, offers a date range of c. 510-530 AD. In the view of the historian Guy Halsall:

Gildas' intent in his writing was to preach to his contemporaries after the manner of an old testament prophet, not to write an account for posterity. Thus, he gives historical details where it serves his purpose; for instance he offers one of the first descriptions of Hadrian's Wall and perhaps the Antonine Wall, though his account of their history is highly inaccurate. However, he omits details where they do not contribute to his message; he is consistently vague, giving few names and no firm dates. Nonetheless, De Excidio remains an important work not only for medieval history but also for British history in general, as it is one of the few works written in Britain to survive from the 6th century.

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