The short answer
There is no certain evidence that a King Arthur of the kind described in the romances ever existed. Camelot, the Round Table, Merlin, Guinevere and the sword Excalibur all come from texts written centuries after the period they describe. What survives from the early sources is a handful of brief and disputed mentions of a war leader named Arthur who may have fought against the Anglo-Saxons. Historians disagree about whether he was a real person, a composite, or a legendary figure attached to real battles.
The post-Roman background
Roman rule in Britain ended in the early fifth century. Over the following generations, Britain broke up into smaller kingdoms and war bands, and groups from across the North Sea settled in the east and south. Written sources for this period are very thin. Much of what we know comes from archaeology and from a few later texts. This was the time of the fall of Rome in the west, and Britain’s fate is among the least documented parts of it.
What the early sources say
The earliest relevant writer is Gildas, a British churchman writing probably in the sixth century. He describes a British victory at a place called Mount Badon, but he does not name the victor, and he does not mention Arthur at all. Neither does Bede in the eighth century.
The first clear reference comes in the Historia Brittonum, a Latin compilation traditionally linked to a writer named Nennius and dated to around the 820s or 830s. It says that Arthur fought alongside the kings of the Britons as their leader in battle, and lists twelve battles, the last at Mount Badon. The list is of uncertain origin, and the work includes much legend.
The Annales Cambriae, the Welsh Annals, include two entries in some versions: one for Badon, in which Arthur is said to have carried the cross for three days and nights, and one for the strife of Camlann, in which Arthur and Medraut died. These annals were compiled in the tenth century, much later than the events, and their dates are not reliable.
There are also hints in early Welsh poetry, which comes from the small Celtic kingdoms of the north and west. A passage in the poem Y Gododdin compares a warrior to Arthur, though the date of that passage is debated, and some scholars place it later than the rest. Welsh tales such as Culhwch and Olwen present Arthur as a leader with a band of marvelous companions, a very different figure from a historical commander.
Geoffrey of Monmouth
The legend we know today took shape in the 1130s, a few generations after the Norman Conquest, when Geoffrey of Monmouth wrote the Historia Regum Britanniae (History of the Kings of Britain). He presented it as a translation of an ancient British book. Most modern historians doubt that and regard the work as largely his own invention built on older scraps. He gave Arthur a conception through Merlin’s magic, a great court, wars against the Romans on the continent, and a final battle at Camlann. He introduced Merlin, Uther Pendragon and Guinevere as figures in the story, along with a sword called Caliburnus, the ancestor of Excalibur.
The book was immensely popular. Medieval kings found it politically useful, since it gave Britain a glorious past. Edward I is said to have visited a supposed tomb of Arthur at Glastonbury in the 1270s, which shows how seriously the legend could be taken in politics.
From chronicle to romance
After Geoffrey, French and then other writers took the story in new directions. Wace added the Round Table. Chrétien de Troyes, in the later twelfth century, wrote romances with knights such as Lancelot and Gawain, centered on adventure and love. Later prose cycles and Sir Thomas Malory’s fifteenth-century English compilation added further details.
Along the way, the king himself became less important to the stories than his knights. Arthur often remains a rather passive figure at his own court. The setting also changed from a post-Roman Britain of war bands to a world of castles, tournaments and medieval chivalry, which was the world of the writers’ own time.
The Glastonbury discovery
In the 1190s, monks at Glastonbury Abbey claimed to have found the grave of Arthur and Guinevere. Many historians suspect the discovery was a convenient find, since the abbey needed money after a fire, though that is an interpretation, not a proven fact. The story gave Glastonbury a lasting link to the legend.
Could there have been a historical Arthur?
Some scholars think a real military leader of the late fifth or early sixth century lies behind the stories. Others argue that Arthur began as a figure of folklore and was later attached to historical battles. A few suggest several different individuals. No inscription, coin or contemporary document names him, and archaeology has not settled the question.
The honest position is that it cannot be decided with the evidence we have. What is certain is that the legend, as we know it, is the work of medieval writers, and that it grew over centuries. Understanding that growth is more rewarding than hunting for a single true Arthur.