By the late fourth century, Germanic peoples had already settled along Britain's east coast and up the Thames river as far as Oxfordshire. By the mid fifth century, they had colonized most of east Britain (the fertile lowlands), leaving only Wales, Cornwall, and Devon as well as two independent British kingdoms in Strathclyde and Elmet.

Arthur was responsible for driving the Saxons back toward the coasts and containing them for the duration of his reign in the early and mid sixth century, but after his death, the British chieftains fought among themselves as well as against the Germanic tribes. By 576, Britain was under Anglo-Saxon domination.