The Real Life History That Gave Rise to Arthurian Mythology
In the early fifth century AD, the once mighty but now seriously troubled Roman Empire found itself under massive pressure from barbarian invaders on multiple fronts. So the Romans withdrew their forces from the far off province of Britain, to use them in an attempt to hang on to territories that they viewed as more vital. It is unclear if the Roman authorities at the time thought that the withdrawal from Britain was permanent. It is possible that they might have considered it to be a temporary pullback, and planned to return once things had settled down.
As it turned out, the legions never returned, and Roman Britain was left on its own for good. The Romano-Britons were beset by invaders, most significantly the Picts who attacked from Scotland, and Saxons who struck from across the North Sea. In what turned out to be a bad idea of epic proportions, the locals, perhaps with the logic that it takes a thief to catch a thief, decided to hire Saxon mercenaries and settle them in Britain, to defend them from other Saxons and similar barbarians. As seen below, it did not turn out well. However, somewhere in the resultant mess was born the legend of perhaps the greatest fictional monarch ever, the mythology of King Arthur. So there was at least that silver lining.