King Arthur is a legendary British leader of the late 5th and early 6th centuries, who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the early 6th century. The details of Arthur's story are mainly composed of folklore and literary invention, and his historical existence is debated and disputed by modern historians .
The popularity of Geoffrey's Historia and its other derivative works (such as Wace's Roman de Brut) is generally agreed to be an important factor in explaining the appearance of significant numbers of new Arthurian works in continental Europe during the 12th and 13th centuries, particularly in France. It was not, however, the only Arthurian influence on the developing "Matter of Britain". There is clear evidence for a knowledge of Arthur and Arthurian tales on the Continent before Geoffrey's work became widely known (see for example, the Modena Archivolt), as well as for the use of "Celtic" names and stories not found in Geoffrey's Historia in the Arthurian romances. From the perspective of Arthur, perhaps the most significant effect of this great outpouring of new Arthurian story was on the role of the king himself: much of this 12th-century and later Arthurian literature centres less on Arthur himself than on characters such as Lancelot and Guinevere, Percival, Galahad, Gawain, and Tristan and Iseult. Whereas Arthur is very much at the centre of the pre-Galfridian material and Geoffrey's Historia itself, in the romances he is rapidly sidelined. His character also alters significantly. In both the earliest materials and Geoffrey he is a great and ferocious warrior, who laughs as he personally slaughters witches and giants and takes a leading role in all military campaigns, whereas in the continental romances he becomes the roi fainéant, the "do-nothing king", whose "inactivity and acquiescence constituted a central flaw in his otherwise ideal society".