The Entire History of Viking Britain
When the Romans officially pulled out of Britain in the 4th and 5th centuries AD they left in their wake a patchwork of Brethonic and Romano-British kingdoms scattered throughout the island just like on the mainland. However it didn't take long for war-like peoples on the peripheries to move in and take advantage of the new lack of an imperial army.
Disclaimer: This summary might be generated by an AI.