The legendary Arthur developed as a figure of international interest largely through the popularity of Geoffrey of Monmouth's fanciful and imaginative 12th-century Historia Regum Britanniae (History of the Kings of Britain). Some Welsh and Breton tales and poems relating the story of Arthur date from earlier than this work; in these works, Arthur appears either as a great warrior defending Britain from human and supernatural enemies or as a magical figure of folklore, sometimes associated with the Welsh Otherworld, Annwn. How much of Geoffrey's Historia (completed in 1138) was adapted from such earlier sources, rather than invented by Geoffrey himself, is unknown.
Although the themes, events and characters of the Arthurian legend varied widely from text to text, and there is no one canonical version, Geoffrey's version of events often served as the starting point for later stories. Geoffrey depicted Arthur as a king of Britain who defeated the Saxons and established an empire over Britain, Ireland, Iceland, Norway and Gaul. Many elements and incidents that are now an integral part of the Arthurian story appear in Geoffrey's Historia, including Arthur's father Uther Pendragon, the wizard Merlin, Arthur's wife Guinevere, the sword Excalibur, Arthur's conception at Tintagel, his final battle against Mordred at Camlann and final rest in Avalon. The 12th-century French writer Chrétien de Troyes, who added Lancelot and the Holy Grail to the story, began the genre of Arthurian romance that became a significant strand of medieval literature. In these French stories, the narrative focus often shifts from King Arthur himself to other characters, such as various Knights of the Round Table. Arthurian literature thrived during the Middle Ages but waned in the centuries that followed until it experienced a major resurgence in the 19th century. In the 21st century, the legend lives on, not only in literature but also in adaptations for theatre, film, television, comics and other media.
The historical basis for the King Arthur legend has long been debated by scholars. 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. The first datable mention of King Arthur is in a 9th-century Latin text. The Historia Brittonum, a 9th-century Latin historical compilation attributed in some late manuscripts to a Welsh cleric called Nennius, lists twelve battles that Arthur fought. These culminate in the Battle of Mons Badonicus, or Mount Badon, where he is said to have single-handedly killed 960 men. Recent studies, however, question the reliability of the Historia Brittonum.
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. These details have often been used to bolster confidence in the Historia's account and to confirm that Arthur really did fight at Mount Badon. Problems have been identified, however, with using this source to support the Historia Brittonum's account. The latest research shows that the Annales Cambriae was based on a chronicle begun in the late 8th century in Wales. Additionally, the complex textual history of the Annales Cambriae precludes any certainty that the Arthurian annals were added to it even that early. They were more likely added at some point in the 10th century and may never have existed in any earlier set of annals. The Mount Badon entry probably derived from the Historia Brittonum.
This lack of convincing early evidence is the reason many recent historians exclude Arthur from their accounts of sub-Roman Britain. In the view of historian Thomas Charles-Edwards, "at this stage of the enquiry, one can only say that there may well have been an historical Arthur [but ...] the historian can as yet say nothing of value about him". These modern admissions of ignorance are a relatively recent trend; earlier generations of historians were less sceptical. Historian John Morris made the putative reign of Arthur the organising principle of his history of sub-Roman Britain and Ireland, The Age of Arthur (1973). Even so, he found little to say about a historical Arthur.
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).
Historical documents for the post-Roman period are scarce, so a definitive answer to the question of Arthur's historical existence is unlikely. Sites and places have been identified as "Arthurian" since the 12th century, but archaeology can confidently reveal names only through inscriptions found in secure contexts. The so-called "Arthur stone", discovered in 1998 among the ruins at Tintagel Castle in Cornwall in securely dated 6th-century contexts, created a brief stir but proved irrelevant. Other inscriptional evidence for Arthur, including the Glastonbury cross, is tainted with the suggestion of forgery. Although several historical figures have been proposed as the basis for Arthur, no convincing evidence for these identifications has emerged.