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Nennius's opus, thought rather short, is epic. He documents the history of Britain from Adam (every decent history should start there, right?) until 9th century. He traces the lineages of the peoples of Europe, talks about migrations and conquests... well, collects everything known to him, I guess :) But his focus is on Britain, first on its conquest by the Romans, and then on the Saxon invasion.
It was very interesting to read about numerous Roman undertakings in Britain and remember the facts I heard at British history lessons in the university. Then I was reminded of the fact that the Britons actually invited Saxons to live on their shores as a buffer from invasions. Who knew they would get OUT OF CONTROL? :) King Vortigern who invited them is an awesome character: he's portrayed as a coward, a tyrant and the one who would sleep with his own daughter and give the resulting (grand)son away. After becoming a hated figure for allowing Saxons take over he seeks refuge in Wales. Nice guy, eh?
As was usual at that time, Historia Brittonum contains not only more or less dependable facts, but also legends and rumors. Some of them are Christian, as for example the miracle of St. Germanus, and some of them are inherited from Welch mythology, as the story of Arthur's boar hunting with his legendary dog. The chronicle ends with the descriptions of the curiosities found in Britain beginning with hot springs in Bath and ending with flying stones.
Historia Brittonum is largely regarded as the first written source to document the life of king Arthur. However, he takes very little place in it, as he is only mentioned as a guy who battled Saxons twelve times. Not that is was something unusual at that time, but he was surprisingly successful in all of them. That's basically all that we get to know about him from this chronicle. He doesn't even fit in the story very well, because for every other known ruler Nennius gives his lineage from a god, or a Roman emperor, or some such famous character, and Arthur gets no such background.
In my book:
A nice example of an early history, although not very informative. Reads more or less as a legend collection, with the addition of some long tiring lineages.