One of my first big lessons in graduate school was that sources, no matter how much useful information they have and despite how early they were written, can never be taken at face value. Take the Historia Brittonum, for instance. Written in about 829, probably by a monk named Ninnius (as Professor Dumville has educated us, he was not named Nennius), in Gwynedd. In the preface he says that he has simply compiled everything he could find into a history. It is an attractive confession. The history provides a complete history of the British people from their first immigration and through the Roman withdrawal. It then goes into great detail about Vortigern, Hengest and Horsa, St. Germanus, and several other individuals and kingdoms of the post-Roman period, providing a wonderful background for the British of the period.
The problem is, people in the ninth century didn’t write histories just to write them. Generally speaking, histories were politically motivated. In 829, King Rhodri Mawr was in his fourth year as the ruler of Gwynedd. His accession had marked the foundation of a new dynasty, and the history served to solidify his position by strengthening the kingdom’s history.
Further than that, the Gwynedd of the ninth century was the most powerful British polity. In the wake of continuing Anglo-Saxon incursions, Rhodri saw it as Gwynedd’s responsibility to unite the British kingdoms against them. Seen from that perspective, the history’s choice of entries makes consistent sense with a sensible goal; when united the British were unstoppable, when divided they were vulnerable. To that end there is an entire chapter devoted to Urien, who is credited with leading an alliance of prominent British kings against the Anglo-Saxons much like Rhodri hoped to. Cadwallon, too, is mentioned as uniting many of the British kingdoms and allying with Mercia in attaining supremacy in the north. A descendant of Urien is recorded there as attempting to usurp the Northumbrian crown.
It showed support of its own kingdom with a chapter devoted to the original foundation of Gwynedd, demonstrating a connection with Rome, a legitimate conquest of a foreign power, and the actions of strong early kings. The history showed support for Powys in the St. Germanus chapter, where the saint himself was credited with legitimizing the foundation of the Powysian dynasty.
Just from the above background it’s clear that Ninnius lied; he didn’t just throw together anything he could find. Even the original entries about Vortigern were put there for a reason; Rhodri Mawr married into his dynasty.
But the original history was not the final draft. The Historia Brittonum was rewritten in the tenth century, again for political reasons. At that time Dyfed was the dominant British power and her king wasn’t as much interested in a British alliance as a Dyfedian empire. Dyfed wasn’t interested in conquering one, either. It’s rulers instead used political tools to attain their ends. They were clever to, never attacking Gwynedd directly as their dynasty was descended from it. They also didn’t have a new history written, they edited the old one to their advantage.
The new version began by attacking Powys, whose territories they coveted. It was in the Dyfed version where Vortigern was first attacked. Following Bede he was blamed for the Anglo-Saxon invasion. Here, though, he was additionally accused of allowing his lust for a woman to lead him into giving away Kent, slept with his daughter, allowed his son to fight off the Anglo-Saxons as he cowered, and finally called for the death of a fatherless child before the great hero Ambrosius showed up and defeated him.