From the historia brittonum, as attributed to Nennius and translated by Giles

In some ways, this is going to be a rather long entry. In some others, it will be rather short. I decided to repeat a section of the historia brittonum (HB) in full, because it will make referencing them easier. But that advances the word count considerably, so I tried to keep my additions short. In light of all I already wrote here, of course I gave a full account of my sources, not claiming any of it as my own. As is to be expected, the details vary slightly with the translator, and the source manuscript, but not enough to warrant a discussion of each.

A picture from the Historia Brittonum, ca. 799, Gwynedd edition

 At that time, the Saxons greatly increased in Britain, both in strength and numbers. And Octa, after the death of his father Hengist, came from the sinistral part of the island to the kingdom of Kent, and from him have proceeded all the kings of that province, to the present period.

Then it was, that the magnanimous Arthur, with all the kings and military force of Britain, fought against the Saxons. And though there were many more noble than himself, yet he was twelve times chosen their commander, and was as often conqueror.

The first battle in which he was engaged, was at the mouth of the river Gleni.[73] The second, third, fourth, and fifth, were on another river, by the Britons called Duglas,[74] in the region Linuis. The sixth, on the river Bassas.[75] The seventh in the wood Celidon, which the Britons call Cat Coit Celidon.[76] The eighth was near Gurnion castle,[77] where Arthur bore the image of the Holy Virgin,[78] mother of God, upon his shoulders, and through the power of our Lord Jesus Christ, and the holy Mary, put the Saxons to flight, and pursued them the whole day with great slaughter.[79] The ninth was at the City of Legion,[80] which is called Cair Lion. The tenth was on the banks of the river Trat Treuroit.[81] The eleventh was on the mountain Breguoin, which we call Cat Bregion.[82] The twelfth was a most severe contest, when Arthur penetrated to the hill of Badon.[83] In this engagement, nine hundred and forty fell by his hand alone, no one but the Lord affording him assistance. 

In all these engagements the Britons were successful. For no strength can avail against the will of the Almighty. The more the Saxons were vanquished, the more they sought for new supplies of Saxons from Germany; so that kings, commanders, and military bands were invited over from almost every province. And this practice they continued till the reign of Ida, who was the son of Eoppa, he, of the Saxon race, was the first king in Bernicia, and in Cair Ebrauc (York). 

Wikipedia 

Considering the the size of the HB Arthur makes up only a small of it. Yet even in this early incarnation it touches on many vital aspects of his legend. First of all, there is the ever present thread of the Saxons. The Saxon invasion of Great Britain remains the subject of debates among scholars, and in preparing for this entry I read a great deal about them, though they remain elusive. For one thing, it is not clear whether the Saxons invaded or migrated or when and how (even if) they mingled with the residents Celts, nor is the time frame clear. We can conclude, at least for now, that Saxons came to Britain, and that they were perceived as a threat to the established order.

It's important to note that Arthut was not chosen as (High) King, but as commander, or Dux Bellorum, which literally translates to war leader. Consistent is the great reverence towards him and that he has to gain his position, although this time it seems to be thrusted on him. The number 12 is also siginfiant in that context, but we will get to that later.

Let it suffice to say that most of the battles aren't identifiable or placeable on the map, but one thing stands out: The battle at Gurnion castle. It's description bears semblance to the battle at Mount Badon, as reported in Annales Cambriae (AC). Since the AC is older, some suggest that the battle made it's way into it from the HB, and that the site was changed because it was more readily recognisable. While the battle of Mount Badon is widely accepted as fact, Arthur is not linked to it before the year 820, which contrasts starkly with his supposed pivotal role in it.

All in all, the HB is strongly doubted by most, and we will further look into it in future entries. Aside from its historical (in?)significance, it's a lesson in how the past becomes distorted in retrospect, and how a name might be attached to something it wasn't meant to, and is in consequence robbed of it's weight. Very cryptic, I know, and I wasn't even talking about Arthur himself. Stay tuned!