Concerning a historical Arthur, there are bits, pieces, scraps, and speculations that point to a British warlord that gained more power due to the fact that the Anglo-Saxons had taken all of Western Britian. Theoretically, this warlord had ties to Rome (maybe not good ones, but way better than anyone else at the time in Britian). At one point the Western Emperor asked the British king to bring an army to the aid of the Romans, and this army actually did leave Britian, made it part way across France I think. This is, however, a theory based on all the precious few documents we have from the era, and I got it from the History Channel's documentary, great stuff btw.