The historical basis for the King Arthur legend has long been debated by scholars.
Hundreds of years, in fact.
Still is.
We do not know who Arthur was, if he was a king, if he was a Roman, or if he ever existed.
One school of thought, citing entries in the Historia Brittonum (History of the Britons) and Annales Cambriae (Welsh Annals), sees Arthur as a genuine historical figure, a Romano-British leader who fought against the invading Anglo-Saxons sometime in the late 5th to early 6th century.
In 1924 it was suggested that the character of King Arthur was ultimately based on one Lucius Artorius Castus, a career Roman soldier of the late 2nd century or early 3rd century. This was suggested that the Arthurian legends were influenced by the nomadic Alans and Sarmatians that settled in Western Europe in Late Antiquity.
All that is known about Artorius’ life comes from two Latin inscriptions discovered in the 19th century in Podstrana on the Dalmatian coast. After a long and distinguished career in the Roman army as a centurion and then primus pilus, Artorius was promoted to praefectus legionis of the VI Victrix, a unit that had been stationed in Britain since c. 122 AD and headquartered at Eboracum (York).
After this (and no doubt due to his long, loyal service to Rome) he became civilian governor (procurator centenarius) of the province of Liburnia, where he seems to have ended his days - likely at an advanced age – and was buried
King Arthur, the 2004 film directed by Antoine Fuqua, with Clive Owen as the title character, Ioan Gruffudd as Lancelot, Keira Knightley as Guinevere, and Ivano Marescotti as Bishop Germanus was based on this premise.
http://en.wikipedia.org/wiki/King_Arthur_(film)
The other text that seems to support the case for Arthur's historical existence is the 10th-century Annales Cambriae, which also link Arthur with the Battle of Mount Badon. The Annales date this battle to 516–518, and also mention the Battle of Camlann, in which Arthur and Medraut (Mordred) were both killed, dated to 537–539.
Some scholars argue that Arthur was originally a fictional hero of folklore – or even a half-forgotten Celtic deity – who became credited with real deeds in the distant past.
They cite parallels with figures such as the Kentish totemic horse-gods Hengest and Horsa, who later became historicised.
Bede ascribed to these legendary figures a historical role in the 5th-century Anglo-Saxon conquest of eastern Britain.
It is not even certain that Arthur was considered a king in the early texts. Neither the Historia nor the Annales calls him "rex": the former calls him instead "dux bellorum" (leader of battles) and "miles" (soldier).
http://en.wikipedia.org/wiki/Historical_basis_for_King_Arthur
Personally, I want to believe that he was a great and mighty Celtic warrior and ruler that once lived in the area of what is modern say Wales.
That Avalon exists just the other side of the mists and the Round Table sits inside Camelot filled with the likes of Sir Galahad, Sir Gawain, Lancelot, and Tristan .... and that they were real "knights in shining armor".
And that Arthur Pendragon was and is the Once and Future King.