Was Arthur a Roman? It is certainly one of the most vexing questions simply because it can be easily and pleasantly answered both yes and no.
The rise and success of Arthur can be ascribed to the loss of the Roman presence and its consequent defense against the Saxon invasions. Into the void that the Romans left stepped Arthur, uniting the Britons against the Saxons. In this sense, he was the Dux Bellorum, the Duke of Battles. (Indeed, some think he should be termed the Comes Bellorum, since he led a mobile cavalry force.)
Now, the Romans had a Dux Brittanum, who was in charge of the Roman defense in the North. Perhaps Arthur inherited his title in the Roman tradition. Nennius, one of the earliest of Arthurian historians, called Arthur Dux Bellorum. In his eighth century manuscript, Nennius wrote of Arthur’s 12 great victories over the Saxons. From Nennius’s book, the Historia Brittonum, we get the story of Badon Hill, Arthur’s greatest victory, which stemmed the Saxon tide for generations. Winning a great battle that set your enemy back for decades was a Roman thing to do.
It can also be convincingly argued that Arthur himself was a Roman. The Romans had been in Britain for four hundred years or so when Arthur was born. It is quite possible that his bloodlines were more than a little equipped with Roman stock. Indeed, one entire tradition holds that Arthur comes from the Romanized name Artor or Artorius. One source has it that Arthur was indeed a Roman, Lucius Artorius Castor, who won many battles and fought a civil war with Medrautus Lancaerius, his designated heir. Yes, Medraut is another name for Mordred in the Arthurian chronicles.
However, if we accept that Arthur was the leader of a mobile cavalry force, then we might find it difficult to accept Arthur as a true Roman. The Roman armies, you will recall, were still wedded to the legion, long after the enemies of Rome had moved on to other tactics. The Romans were, of course, experimenting with cavalry; but, as with other aspects of the empire, the Romans preferred to stay the course, as it were. So, in this respect, was Arthur a Roman? Maybe. If he was, then he was very much a British Roman.
We have observed a bit of Nennius. One of the other famous early British writers was Geoffrey of Monmouth, whose History of the Kings of Briton was one of the most influential books in the history of Arthurian studies. From this book we have the first full picture of Arthur, legends or not. From this book we have the first full picture of Merlin, legends or not. Geoffrey’s place in history is that of first chronicler to the masses. His tales formed the basis of Arthurian histories and legends for hundreds of years. What does Geoffrey say about Arthur’s being a Roman? Not much, except to say that Arthur defeated the Roman army commander, Lucius Hiberius, at Saussy, in the heart of Europe. Could this have been a civil war for the throne of the empire? One of the stories about immediately post-Roman Britain is that Rome was forever searching for a Restitutor Orbis, a world restorer. The Empire was beset from without and within. In 410, Alaric and the Visigoths sacked Rome. Fifteen years earlier, the Empire had been split in two. The Romans had left Britain altogether because they were needed at home; the threats from Germany were too grave. The need for a Restitutor was enormous. This was perhaps the basis for the Arthur story as well, for Britain, without the Roman defensive presence, was susceptible to Saxon and Jute and Angle invasion, much the same as Rome was susceptible to Goth and Visigoth and Hun invasion. Arthur, the Roman soldier, would save the world of Britannia from the Germanic menace. Geoffrey carries the story further and has Arthur save the Roman world from itself. Even Arthur’s wife is Roman in Geoffrey. (Her name is Ganhumara.)
What then is the answer? As we can see, we don’t have one. The information just isn’t out there. More to the point, we just haven’t found it yet. Arthurian experts have their theories, but no one knows for sure. Arthur could very well have been Artorius. He could have been someone entirely different.