At some point in the early sixth century, Gildas wrote a letter known as the De Excidio Britanniae. It is one of only two direct sources of the period.
The letter, however, is not a lecture or a sermon, as was assumed for centuries. Nor was it written, in a period where the maximum life expectancy was 55, for people who would have been able to debate a vast majority of his history. Study by experts in Roman culture have determined the epistle was written instead as a legal argument. In traditional Roman fashion, it lays out a history streamlined toward making his point; in this case that the British people will suffer the wrath of God if they do not return to their proper moral and ethical ways. After relating his history up to the present day, he then attacks five kings. This is followed by a broader tirade against his peers.
The focus of his attack, however, is not what is important here. The fact that his purpose is other than writing a history is. It means that he has, by necessity, modified his history to fit in with his own agenda. He claims that an Ambrosius, of Roman descent, rallied the British. But we cannot know if Ambrosius was a national leader or just some local or even mythical hero who fit into Gildas’ mold of the right leader without corroboration. He does not mention Arthur, but is that because Arthur did not exist or because he did not measure up to Gildas’ standards. There is even a legend that Arthur killed Gildas’ brother; reason enough to omit him. More poignantly, Gildas says that the most immoral of all his kings invited the first Germanic peoples onto the island. Did the king exist or is he a convenient powerful figure? Did the Germanic leader exist, or is Gildas here using his debating skills and giving his audience a focal point? Most importantly, did such an agreement take place at all?
Gildas gives very little information and what he does offer is often tainted by his argument’s perspective. He is the most tempting historian in the post-Roman period and has almost nothing to offer in the way of useful facts.