Whether King Arthur existed or not is doubtful. However if King Arthur did exist, then he would have lived sometime between 400 AD and 600 AD, a time of turmoil in Britain following the Roman withdrawl. And a time when written literature did not exist, therefore events during this period are only known about from folklore passed down several generations before being written down, or from modern archeology giving insights from excavations of sites.
If there was ever a true King Arthur in history, he would probably be Romano-British warleader, probably named Artorius, which is a Roman name for Arthur. Though the Roman legions may have left Britain in AD 410, the general population of mixed Romans and Celts, would have had generations of Roman law, education, culture and way of life.
The name, Artorius, is similar enough to the Gallic god of the bear, Artaius or Artaios. The Roman had identified this god with their Mercury. In Latin, Arto means "bear". So Arthur like other Welsh characters, could be derived from ancient Celtic god in Gaul (France).
Possibly the earliest reference about come from Y Gododdin written by the Welsh poet, Aneirin, c. 6th century. Though, the poem was attributed to the 6th century, Gododdin was actually preserved in the manuscript called Book of Aneirin, in c. 1250. The poem only mentioned Arthur's name, once, referring to a warrior in the poem as being brave "but he was no Arthur". There is no linguistic evidence that would dati Y Gododdin as a whole before the 9th- or 10th-century
Most of the early legends of King Arthur come from Welsh sources, between the 8th and 10th century.The earliest tale where King Arthur had more active role in early Welsh literature come from Culhwch and Olwen (before AD 1100), one of eleven tales in the Mabinogion. Other tales found in the Mabinogion were composed of later date from Dream of Rhonabwy and the three Welsh romances: Geriant, Owein and Peredur. These tales are similar to those found in Chretien de Troyes' three King Arthur romances – Erec, Yvain and Perceval, which may have been composed earlier than the Welsh versions.
The case for a historical King Arthur rests on two sources, the Historia Brittonum and the Annales Cambriae, both of which show that King Arthur that is at least partly historical. The Historia Brittonum was written anonymously in A.D. 829/30, the ascription to one 'Nennius' now being regarded as false. Historia Brittonum is of dubious historical value as it shows a number of mythical figures as genuinely historical, but it does show at least that King Arthur as a figure was current by A.D. 829/30 at the latest.
St Bede the Venerable wrote in his Historia ecclesiastica gentis Anglorum ("Ecclesiastical History of the English People"), in AD 731, but does not mention King Arthur by name.
The Annales Cambriae, was compiled in 950s and is sometimes seen as offering evidence for Arthur being a historical figure. It mentions Arthur in two entries: that for A.D. 516 which tells of 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 the victors" and that for A.D. 537 concerning "the battle of Camlann, in which Arthur and Medraut fell". The Annales Cambriae is basically a version of the 'Chronicle of Ireland' and there is no justification for thinking that any of the British entries are drawn from contemporary or even near-contemporary sources and, rather, they should be seen as interpretations dating from between the very late 8th-century and the mid 10th-century when the Annales reached its final form.
In Gildas's De Excidio Britanniae, Ambrosius is given prominence as the initiator of the British counterattack which, after the fighting of several battles, culminates in the battle of Badon.
These sources cannot prove that there was a historical 5th-/6th-century King Arthur and no contemporary or near-contemporary source makes any mention of him.( The best we can say is that there existed by the 9th-century at the latest a concept of King Arthur as a historical figure. The sources that we have today are simply not of the quality that would allow us to come to any firmer conclusion than this.
Geoffrey of Monmouth derived his sources, mainly from Nennius, but also from the Gildas, Bede and the Annales Cambriae. However, Geoffrey set the year of Arthur's deaath a little later on 542. Also, Geoffrey had turned Ambrosius Aurlianus into Aurelius Ambrosius, an uncle of King Arthur. Around A.D. 1139 Geoffrey of Monmouth (Galfridus Monemutensis) completed his Historia Regum Britanniae ('History of the Kings of Britain') which glorified King Arthur and made him an international warlord. This work became influential throughout western Europe and affected the King Arthur legend in all areas with the result that scholars look to sources written before Geoffrey's Historia for the 'original' King Arthur.
Geoffrey of Monmouth wrote about the warrior king of Britain at the time of flux in Britain. After the Romans left, Saxons and Angles had invaded Britain, driving the Britons (Celts) into Wales, Scotland, Cornwall and Brittany between the 5th and early 7th century. The Normans from Normandy in France won control of Britain after the Battle of Hastings in 1066. Geoffrey was writing at the time of the death of Henry I and in the reign of King Stephen (1135-1154), a period of anarchy and civil war. It could have been that Geoffrey wanted to give them a British hero, someone that the country could be proud of, and massaged the historical scripts that existed, to get King Arthur as his hero.
The souces that are used for King Arthur research are as follows :-
Gildas - died 570 AD
The Venerable Bede - 731 AD
Nennius - 800 AD
Geoffrey of Monmouth - 1130 AD
Chretien de Troyes - 1170/1185 AD
Gerald of Wales - 1145 to 1223 AD
Medieval Welsh Literature - 8th to 14th cent.
Medieval French Literature 1155 - 1235 AD
Sir Thomas Malory 1416 - 1470
John Leland 1545
Alfred, Lord Tennyson 1809-1892
T.H.White 1906 - 1964
The Legend of King Arthur