Obviously with pre historical things we have to use our imagination since evidence is either scant or non-existent. But history now seems to accept more retconning and re-interpretations than before. Some re-interpretations do make sense when new evidence is found, but there are times when it’s done more for new narratives than actual truth seeking.
The Angles – ancestors of the English were the people whom he was fighting against.
Now was Arthur Roman or Celtic or Welsh etc is up to debate.
The more typical question would be "is Arthur based on any historical events at all?"
But the further back you go, the less historical they become. And I don't mean in the sense of historical records being less reliable farther back in time; I mean that the characters in the oldest legends are clearly supernatural figures. There is no reason to postulate supernatural figures as having historical enemies. Why would they?
Most of the Arthur "myth" is deliberately constructed fiction by specific authors, rather than folk myths.
I only recently started to understand that the post-Roman "dark age" was not quite as dark as I thought it was. Scholars and other interested parties can tease out a fair amount of knowledge of these times. Even though it gets quite obscure.
I have a recent book by Caleb Howells about "King Arthur" and he goes into some interesting detail about some of this stuff. He also has a YouTube channel. There is real history to be discovered here still (but much can only ever remain speculative unfortunately).