The short answer
Historians debate whether a real person lies behind King Arthur, and there is no way to settle it with the evidence we have. No source written during Arthur’s supposed lifetime, in the decades around 500, names him. The knights, the Round Table, Camelot, Guinevere and the sword in the stone all come from writers working centuries later. What survives from the early period is a small group of brief, late and sometimes contradictory references.
The setting: Britain after Rome
Roman rule in Britain ended in the early fifth century. In the following generations, Germanic settlers (the groups later called Anglo-Saxons) established themselves in the east and south, while British-speaking kingdoms continued in the west and north. Written records from this period are scarce, which is the main reason the era is so hard to reconstruct. Any real Arthur would belong to this world of local war leaders, not to the castles and tournaments of later romance.
What the early sources say
Gildas
Gildas, a British churchman writing probably in the sixth century, described a British victory at a place called Mount Badon. He does not name Arthur. He credits an earlier leader, Ambrosius Aurelianus, with organizing resistance. The absence of Arthur from the nearest thing to a contemporary account is the strongest argument for caution.
The Historia Brittonum
A Latin compilation usually dated to around the early ninth century, traditionally attributed to “Nennius” though that attribution is uncertain, is the first text to give Arthur a role in the fighting. It describes him as a war leader, often rendered as “leader of battles,” who fought alongside the kings of the Britons, and it lists a series of battles that ends at Badon. The text was put together roughly three centuries after the events, drawing on sources we cannot check, and it also contains clearly legendary material.
The Welsh annals and poetry
A set of Latin annals compiled in Wales, in a form that is also later than the events, includes entries mentioning Badon and a battle called Camlann, in which Arthur and another man are said to have died. These entries were probably written down long after the fact. Welsh poems and tales also mention Arthur, sometimes as a hero with a band of followers. A poem known as the Gododdin has a passing reference to him, but scholars disagree about how early that line is and whether it was added later.
What this adds up to
Taken together, the sources suggest that by the ninth or tenth century some people in Britain believed in an Arthur who was a famous warrior, and that Badon was associated with him. They do not prove that a historical individual fought there. Some scholars think a real commander is likely, some think Arthur began as a figure of folklore, and many say the evidence cannot decide. Attempts to identify him with a particular known person, or to locate Camelot at a particular hill fort, have not won general agreement.
How the legend grew
Geoffrey of Monmouth
Around the 1130s, Geoffrey of Monmouth wrote a Latin history of the kings of Britain that gave Arthur a full career: a conception through Merlin’s magic, conquests across Europe and a final battle against his nephew Mordred. Geoffrey presented this as history, but historians treat much of it as invention or imaginative reworking. It was widely read, and it arrived not long after the Norman Conquest, when the new rulers of England and their neighbors were interested in British past and royal prestige.
Romance writers
Writers in French, notably Chrétien de Troyes in the later twelfth century, moved the stories toward chivalric adventure and love. Lancelot, the quest for the Holy Grail, and courtly rules of conduct entered the tradition at this stage. The Grail material, which is a literary and religious invention, is a separate subject from Arthur’s possible history; the Holy Grail legend had its own development. The idea that Arthur presided over an ideal knightly court says more about the medieval nobility’s picture of itself than about the fifth or sixth century. The romances both celebrated and tested medieval chivalry, and they were a main route by which those ideals reached a wide audience in medieval literature.
Malory and after
In the fifteenth century Sir Thomas Malory gathered many of these French and English tales into a single long English prose work, which became the main source for later English retellings. Because it was among the early books printed in England, it also helped fix a standard version of the story. Later writers, from the Victorian era to modern novelists and filmmakers, have reshaped Arthur again, sometimes as a Roman-British commander, sometimes as a medieval king.
Reading Arthur sensibly
A few points help keep the subject clear.
- Dates matter. Almost everything people picture when they think of Arthur was added at least six hundred years after the era when he is supposed to have lived.
- Absence of evidence is not proof of absence. Written sources from fifth-century Britain are so few that a real leader could have gone unrecorded. That is different from showing that one existed.
- Be wary of confident claims. Books and documentaries that announce that Arthur’s tomb or castle has been found usually rest on speculation.
- The legend is real history of a different kind. Even if there was no historical Arthur, the stories about him are a genuine part of medieval literature and show how people used the past to talk about kingship, loyalty and failure.
The honest summary is that a post-Roman British war leader called Arthur may have existed, that the evidence is thin, and that the king of Camelot is a creation of the later Middle Ages.