Though we are able to establish a history of the Arthurian literature from its beginning to the present day, historians and archaeologists have yet been unable to unearth any reliable evidence about the historic King Arthur. The origin of the name itself is unclear, it might derive from the Latin Artorius or could be of Celtic origin, artos viros, meaning 'bear man'. Our most reliable sources are literary ones which tell a lot taking into account that the early medieval authors had no interest of the depiction of history as a truthful account of events. Myth, legend and propaganda are wrapped around, perhaps the smallest core of historicity. To untangle and peel off these layers of storytelling and straightforward lies seems to be impossible for the modern scholar.
A fine example of the problems at hand is the today ruined abbey at Glastonbury in Somerset. This place began to be connected to the story of King Arthur in the beginning of the 12th century. In 1191 the monks of Glastonbury miraculously discovered the grave of King Arthur and his Queen Guenevere. This might not raise any doubts, unless you know that in 1184 most of the abbey had burned down and the monks were desperately in need of money to pay for the necessary repairs. What greater tourist attraction could you have than the grave of such famous a king and queen. There might also be a political reason to consider, The Welsh were heavily resisting the English at that time and the discovery of the grave of their ancient king who had been prophesied to return as a Celtic champion in time of need could very well have weakened their morale.
The safest assumption we can make nowadays is that Arthur, whether king or not, was a Celtic warlord fighting the invading Saxons in the 5th or 6th centuries. Anything beyond this is much more than mere speculation.