Arthur, historians believe, lived in the 4th and/or 5th centuries. The exact dates depend on the dates given for the most important events in Arthur’s life, mainly the Battle of Badon Hill and the Battle of Camlann-his greatest victory and his final battle, respectively.
As a 4th/5th century warlord or king or battle chieftain or cavalry leader or whatever else he really was, Arthur was a figure of his times. He was a soldier, first and foremost, because his people were at war with the invading Saxons, Angles, and Jutes. He was a powerful man because he commanded the respect and devotion of a large force of men. (Almost no one doubts this point.) He won battles and lost battles. (Most soldiers did both.)
But details? That’s where the sketchy part comes in, for no eyewitnesses to the events in Arthur’s life wrote anything down. Much of what we have as the Arthurian tradition stems from writers writing long after the fact.
The first historical source is Gildas, a monk who was writing at the height of the Saxon invasions. He mentions Badon Hill but does not name Arthur as its champion. That connection is provided by Nennius, another monk, this one writing in the 8th century. The most famous of the medieval sources is Geoffrey of Monmouth, who wrote in the 12th century. Examine those dates again. Arthur lived in the 4th and/or 5th centuries. The sources cited wrote in the 6th, 8th, and 12th centuries, respectively.
They had no eyewitness accounts. They were working from hearsay information-the wonderful oral tradition that gives us vibrant imagery but untrustworthy sources. It can be argued powerfully that Arthur did exist and that he even did a few or some or many of the things he is said to have done. The literature tradition is strong and spans many cultures and centuries. The archaeological records back this up as well. The problem is that as historians, we need facts to prove our theories. And in the Matter of Britain, facts are not always forthcoming.
Perhaps that is part of the charm of the pursuit. The more things get disproved, the harder people look-unlike other pursuits, in which people tend to falter in their drive if the evidence against their hypothesis piles too high. It is probably too much to ask for archaeologists or historians to find a King Arthur Rosetta Stone, the one piece of evidence that solves a centuries-old mystery. And yet, you never know.