Arturus?

I'm not sure what to make of the new movie King Arthur. I mean, I don't claim to be any kind of expert on the subject, but I have done more than my fair share of study of the Arthurian legends, including the historical realities of Arthur, Merlin and the Knights. And nowhere, in any of my readings & viewings, have I ever seen reference to the claim that Arthur was a Roman officer. From what I've gathered, the movie takes the stance that Arthur is a bit of a disillusioned Roman soldier who finds himself leading an alliance of Brittons and Romans against the Saxons in the mid-5th Century. Now, since the Saxons were still in power up until the Norman Invasion (lead by William the Conqueror--one of my direct ancestors, BTW) in the late 11th Century, and ruled all of England under Alfred the Great (another of my direct ancestors, BTW) in the 9th Century, it seems a bit hard to swallow that the Romans & Brittons defeated them in the 5th Century.

In all of my various readings, the leading conclusion (but let's face it, no one knows for absolute sure) is that Arthur was actually an Irish chieftain during the 4th or 5th Century (well, apparently the movie got the time frame right) who united his peoples against the Romans. Quite a bit of evidence also points to Merlin being an actual person (again, an Irishman) around that same time. Additionally, there is a plethora of documented evidence that many of the legends of the Arthurian Knights were actually based on the Fianna, a band of warrior elite guarding the High King of Ireland in the early 4th Century.

So, again, I'm not really sure where the movie makers are coming up with the claim that Arthur was a Roman soldier. Now, i understand that this is just a movie, and of course they're allowed to fictionalize things. After all, most of the Arthurian movies try to portray events taking place in the Middle Ages. But, they're trying to bill this as the "authentic story" of Arthur and the Knights. I'm curious who did their research and where they got their info from.

But, that being said, it still looks like it'll be a pretty good movie, and I'll likely go ahead and see the movie for the pure entertainment value. Of course, the fact that the incredibly lovely and talented Keira Knightley stars is a major plus