If we are to place Arthur in a historical context the first task needs be to identify the period in which he lived. This is given in the AC by the dates of his two most significant battles: the victory at Badon in the year 518 and the defeat at Camlan in 539.
Is it possible to verify this time period? The chronicle of his enemy, the ASC, makes no mention of these battles. This is not surprising as far as Badon is concerned since the Saxons would have preferred their defeats to be forgotten but it is surprising that Camlan is not mentioned, unless this was an internecine battle between the Britons.
Nennius, in his HB provides a clue for the dating of Badon:
“Hengist having died, however, his son Octha crossed from the northern part of Britain to the kingdom of Kent and from him are descended the kings of Kent. Then Arthur along with the kings of Britain fought against them in those days, but Arthur himself was the military commander [“dux bellorum”].”
After the above quote Nennius lists Arthur’s 12 battles, culminating in Badon, and following that the reign of Ida. The ASC states Aesc succeeded Hengist in 488 and so Arthur’s battles occurred sometime after that date. Gildas speaks of a period of relative peace after Badon and says that the interval was of such length that the generation that had known the turmoils had passed away. The ASC indicates after 547, the start of Ida’s reign, there was no state of peace. If we now deduct 30 years, an approximate length of one generation, from the start of Ida’s reign we arrive at a date close to that given by the AC for Badon, namely 518.
Bede in his HE when giving Ethelbert’s genealogy indicates Octa was a son of Oisc and a grandson of Hengist. This contradiction with the HB may be explained by an earlier reference in that document which says that Hengist sends for his son Octa and Octa’s brother Ebissa. I believe the latter person may have been Oisc who was in reality, as Bede states, Octa’s father.
The GRA gives additional information concerning Oisc that he reigned for 24 years. We may, therefore, conclude Octa succeeded him in the year 512. It follows that Arthur’s 12 battles occurred between the years 512 and 518. The Saxons’ catastrophic defeat in the latter year may explain Octa’s absence from the ASC.