I've noticed something odd a few times, that the history of Arthur in film is almost like a reverse of the development of Arthurian literature before then. This may not hold up under a detailed break down, but I know when I first checked IMDB the first several movies to feature King Arthur were adaptations of Mark Twain's A Connecticut Yankee in King Arthur's Court, a book written in 1889, literally just before the Motion Picture industry was born. Then the Golden Age of Cinema tried to be evocative of Victorian depictions of King Arthur, then Excalibur tries imperfectly to mimic the most traditional versions and others followed suit.
Then starting at the turn of the Millennium the trend become depictions that at least put on a pretense of trying to seem like the real History behind the Legend. There are constant references to Rome and Saxons and Hardrian's Wall and Aurelianus Ambrosius and Vortigern, and if they include supernatural elements it's all about outdated popular views of the Druids (they weren't matriarchal or goddess worshipers, sorry Mists of Avalon). Thing is in my view none of those were based on any really good historical research.
I believe an historical Arthur existed in the first half of the 6th Century, his story was a lot different from the popular mythology, but I believe he absolutely did exist.
The lack of reference by Gildas and Bede doesn't mean anything. Both do mention Badon but neither identifies any leader on the Briton side. Bede was writing from an Anglo-Saxon POV, so he named few Briton leaders same as Welsh sources name few Saxon leaders. But Bede does say during the latter 5th and early 6th century the native Britons did push back the Saxons. And Gildas wasn't a historian, his focus was to rant about the present, but gave a brief backstory. He talks about Aurelianus Ambrosius before Badon, but clearly implies a gab in between.
The geographical affiliation Arthur sometimes has with southern regions comes from Geoffrey of Monmouth mistakenly listing the 5 Welsh and Cornish kings Gildas ranted against as Arthur's immediate successors. The pre Geoffrey references, and most of the 7th century individual names after Arthur seem to imply a more northern location.
The three oldest references to Arthur (the only surviving ones that predate 1000 AD) have one interesting thing in common. None of them call him King.
First is the Y Gododdin by Aneirin, It refers to a warrior who "glutted black ravens [i.e., killed many men] on the rampart of the stronghold, although he was no Arthur". (Fletcher, Richard (1989). Who's Who in Roman Britain and Anglo-Saxon England. Shepheard-Walwyn. pp. 17–19.).
The Annales Cambriae has two references to Arthur.
Year 72 (c. AD 516) The Battle of Badon, in which Arthur carried the cross of our Lord Jesus Christ on his shoulders for three days and three nights and the Britons were victors.The carrying the Cross on his Shoulders is likely a welsh translation issue that is meant to say he had a Cross on his Shield.
Year 93 (c. 537) The Strife of Camlann in which Arthur and Medraut fell and there was death in Britain and in Ireland.
Year 129 (c. 573) The Battle of Armterid
This date for Camlann disagrees with Geoffrey and the Brute Tysilo's 542 date, for many reasons I think this date is the correct one. It's also been noted that this earliest reference to Mordred doesn't even tell us he was an enemy of Arthur, they could've been on the same side.
The battle of Armterid doesn't involve Arthur himself but other people who would become linked to Arthur in later traditions. Mainly Myrddin(Merlin) and the sons of Eliffer who I'll mention later.
The most informative but perhaps most controversial of the three is the Historia Brittonum, traditionally attributed to Nennius. I should note that most might disagree with the order I chose to discus these three in.
Nennius not only doesn't call Arthur a King but seems to explicitly define him as not one. He calls him dux bellorum which had been interpreted to mean "War Commander" or "leader of the battles", and says he fought alongside the Kings of the Britons. He also lists Badon Hill as the last of 12 campaigns Arthur fought. Nennius also calls Badon a siege which makes more sense of the Annales Cambriae saying it was three days and three nights.
Gildas and Bede have been interpreted as implying a much earlier date for Badon, around 480-500. It could be there was more then one battle of Badon and there have been many locations with more then one battle named for them like Megido. Or 516 could have been the end of a campaign began 20-30 years earlier. Scholars don't even agree which location Badon refers to. If they are separate battles I think Gildas likely meant a southern maybe Welsh location and the two sources directly linking it to Arthur a northern one.
I think the Historical Arthur was Arthuis of Elmet. Elmet was a Briton location just south of York(Eboracum). Arthuis was called a "Prince" but never a King. His father Mascuid Glof estimated to be born in 440 AD was King of Elmet. As was his brother Llaenawc born about 475 AD. And his nephew Llaenawc's son, Gwallawc Marchawc Trin who was born about 520 AD.
The source I'm going on for these dates seem to imply a lot of these Briton Kings had their successors around 40, which may seem odd at first but David had Solomon when he was over 40, and didn't start having kids till 30.
Arthuis (Arthwys) of Elmet was born about 479 AD. Many Kings have had a brother serve as a right hand man and effectively co ruler. And Since Gwallawc would have turned 17 the year Camlann happened, Arthuis may also have ruled as regent for him if Llaenawc died before then. He could have been the person Llaenawc entrusted to command the army and thus was his "War Commander" at Badon.
The son of Gwallawc was Ceretic of Elmet. Born about 560 AD and died in 617 AD. I think he's almost certainly the Keredic of Geoffrey who began his reign in 555 AD.
It has often been noted that Cerdic of Wessex, an ancestor of Ælfrēd The Great had a Welsh/Briton name, clearly a from of Ceretic/Keredic. The earliest sources on Cerdic are just him being named in the genealogy of Ælfrēd The Great. The biographical information comes later and is iffy, when he lived is disagreed on, and I think his geographical association with Wessex could just be because he's an Ancestor of Ælfrēd The Great.
Many scholars like Kenneth Sisam believe it's beginning with Cerdic that the genealogy is real people and all the names before were made up. I don't agree with that exactly as a fan of Bill Cooper's After The Flood, but I think we see a mingling of a Briton and Saxon royal line. I think Cerdic/Ceretic/Keredic were all the same person. Either Gwallawc or Ceretic himself or both took a Saxon Princess descended from Odin as a wife. I disagree with Cooper on Sceaf being Japheth though, I think the Pross Edda is right on him descending from Memnon and Priam.
The implication of all that is that Ælfrēd The Great may have been a distant nephew of Arthur, thus connecting Arthur to modern British Royalty.
Arthuis of Elmet was not the first Arthuis however. Before him was King Arthuis (Arthrwys) of Eboracum/Ebrauc, who was born about 455 AD. And who's son Eliffer (mentioned above) was born in 473 AD and died in 560 AD. This Arthuis was the son of King Mor born about 420 AD, who was the son of King Ceneu born 382, the son of Coel Hen. Eboracum was the main Roman Military fort of Britan, and Coal Hen estimated to have lived from 350-420 AD seems to have been the last of the Roman Duces Brittanniarum. He may be the King Coel meant by the old nursery rhyme Old King Cole.
Both Arthuis have a common pater-lineal ancestry. Mascuid of Elmet was the son of King Gwrast of Rheged, born about 422 who was the son of Ceneu. Other northern dynasties also descended from Coel Hen who also had a son named Gorbanian. And a daughter of Coel Hen married Cunedda Wledig and became ancestral to the kings of Gwnedd, the princes of Wales and the Tudor dynasty.
A couple early references to Arthur seem like they would chronologically have Arthur of Eboracum (which the Welsh called Efrawg) in mind, they're also the earliest ones to call him a King.
The 11th century Welsh Poem Geriant, son of Erbin presents the title character as a contemporary of King Arthur. This Geriant is Gerren Llyngesoc king of Dummonia (a kingdom in Cornwall) born about 448 BC. The eldest son of King Erbin born about 427 AD. Gerren's son Cado lived from 482 to 537 AD (same year as Camlann interestingly) and was the father of the Constantine who was one of the 5 kings condemned by Gildas and made the direct successor of Arthur by Geoffrey.
And another is the Legenda Sancti Goeznovii, traditionally written in 1019 as a biography of St. Goeznovius. This narrative talks about Vortigern but then ignores Ambrosious and Uther and presents Arthur and reigning right after Vortigern. This source also says Arthur was "summoned from human activity" rather then saying he died, perhaps the first seed of the Avalon tradition.
The other lives of the saints that mention an Arthur are about late 6th and early 7th century saints. So someone like Arthur of Dyfed is more likely there. And that Arthur isn't always portrayed positively.
Timeline wise if Riomathus was someone known to the Britons as an Arthur, then Arthuis of Eboracum works best. But Riomathus most likely ruled either Brittany and/or a part of Britan close to the English Channel. There are also reasons to associate Ambrosious/Emrys with Brittany.
Goeffrey and the Brute Tysillo are already corrupt sources, but I think even they could have some useful info in them, especially in my view details found in the Brute but not Goeffrey. One Brute specific detail is listing a Madoc as a contemporary of Arthur. I agree with the theory that the Prince Madoc who traveled to the Americas lived in the 6th not 12th century as usually stated. This could also be the same as the Madoc who gets remembered in Welsh poetry as a son of Uther, brother of Arthur and father of Eliwold.
If Geoffrey Ashe's theory that Emperor Lucius Tiberius was based on Glycerius is true, that also fits the time-frame of Riothamus. But Geofrey didn't actually call Lucius an Emperor but "Procurator of The Republic" so maybe he wasn't actually The Emperor.
When Justinian was trying to reconquer the Western Empire in the 530s through 550s AD, he did demand the contemporary Frankish King, a son of Clovis, to pay him tribute very similar to what Goeffrey and the Brute say Rome demanded of Arthur. In which context I think Liberius (praetorian_prefect) is a good candidate for Lucius Tiberius.
The grandson of Arthuis of Eboracum was Peredur son of Eliffer, who lived from 510-580 AD, and was King from 560-580. He would have been a contemporary with the entire reign of Arthuis of Elmet, and is thus another possible connection between them.
Peredur son of Efrawg was a medieval Welsh Romance who's relation to Chrétien's Perceval, the first appearance of that character, is debated. Many think it's just a Welsh adaptation of Perceval, but others think both were drawing on an earlier Welsh tradition.
Efrawg as mentioned above was a name for Eboracum/York. Peredur son of Eliffer must be the Peredur that author had in mind whether he was adapting Perceval or not. But the historical Peredur unlike in the romance wasn't an infant when his father died, Eliffer died when he was 50.
Most mysteriously Jesus Collage MS 20 lists an Arthur Penuchel as a brother of Peredur and Gwrgi.
So the Arthur of legend was born from these two Arthuis being confused with each other and merged together. It's interesting that the earliest references to Arthur seem to only make sense with the later of these two.
So that's my theory on the history. I may make more posts in the future on the fiction.
I just read here about a new source of information. The Vita Sancti Dalmatii dated to 800. It doesn't mention Arthur but it is evidence of a Briton presence in northern Burgundisa from 534-541 AD. I think Arthuis could have a lead a campaign, there, some of his forces remained behind when he had to return to the Island to perish as Camlann in 537. This remaining presence could be the reason Geoffrey has his date off by five years.
Update: So it turns out Arthuis of Elmet is just an imagined name on internet websites, like those people who only exist in Mormon genealogies. And even Arthuis of Eboracum is dependent on late medieval genealogies, the oldest one doesn't have him. So this theory lasted less then a day.
I still think Arthur existed and that the Annals Cambraie dates are the ones to go by, and plenty of what I talked above are clues to the riddle. But for now it's still a mystery.