“Forsooth, and Tarrie Not to drinke ye thine Ovaltine”
We can say straight away he wasnt anything to do with stopping the Anglo-Saxons - he was fighting other Britons in the North.
Gildas, a near-contemporary, names Aurelius Ambrosius as the leader of the Britons' resistance to the Saxon invasion. The name "Arthur" doesn't appear until Nennius, who wrote several hundred years later. Legend, however, conflates Arthur with the leader of the resistance to the Saxons. The big conundrum in the King Arthur debate is to somehow connect Ambrosius and the later Arthur.
Was there an historical Arthur? Well ... since the Britons didn't just roll over and surrender to the invading Germanic tribes, someone (or several someones) led the resistance, which enjoyed a temporary success before eventually collapsing. The later Arthur legends coalesced around this heroic figure. It's the name change from Ambrosius to Arthur that causes the difficulty.