Sources for the Arthurian legends differ widely about almost every aspect of King Arthur and his reign. Major historical sources, such as the Anglo-Saxon Chronicle and Bede's Ecclesiastical History, do not mention him at all. Some of the sources which do refer to Arthur, or someone who might be identified with Arthur, do not mention that he was British or lived in Britain. One places him on the Dalmatian coast in Croatia. The sources which do describe Arthur as a Briton cannot agree whether he was from Wales, Scotland, or Cornwall, or even if he was a king.

There are a number of models suggested for Arthur, ranging over hundreds of years from the second century to the sixth. Some of these were actually Romans themselves, such as Lucius Artorius Castus. Others were Romano-Britons, who were culturally Roman: Latin speakers steeped in aristocratic Roman culture, such as Ambrosius Aurelianus. It is significant that not even the earliest sources, or those that place him earliest in history, portray Arthur as a Briton fighting against the might of the Roman Empire. This would be a very natural depiction for a national hero and would place Arthur in the tradition of heroes fighting against the odds, from King David to Robin Hood.

However, the Arthurian tradition is actually the reverse of this, since Arthur is often depicted as culturally Roman. In the Historia Brittonum, one of the most important early sources of British mythology and the first to mention him, Arthur is not a king but a dux bellorum, a Roman officer. The Historia Brittonum also mentions the tradition that Britain is named after Brutus, a descendant of Aeneas, giving the Britons and the Romans a common ancestry. Like the Romanization of Arthur, this shows that the educated classes in Britain saw themselves and Britain as closely allied with Rome and wanted to claim a close relationship with that center of civilization, even in the time of its decline. Geoffrey of Monmouth, who does depict Arthur as a king, relied heavily on the Historia Brittonum as a source.