By the time the Romans left Britain in c. 406 CE, much of modern England and Wales was controlled by Celtic warlords, the first ‘Britons’. In c. 450 CE England was invaded by waves of Germanic peoples (Anglo-Saxons). They were unable to subdue the tribal regions of modern Cornwall and Wales, or Scotland (which was controlled by a string of Pictish statelets). They formed kingdoms in Wessex, Sussex, Kent, East Anglia, Mercia and Northumberland, settling near rivers or the sea, in places that were easily reached by boat. Southern and eastern England were the first regions to ‘fall’, followed by northern England in the 7th century. The Anglo-Saxons incorporated the Scottish borderlands into the powerful kingdom of Northumbria, which peaked in 638 CE under the Christian convert, King Oswiu. In c. 770 Mercia, under King Offa, was the most powerful of the Anglo-Saxon kingdoms and, by the 780s, controlled most of southern England.
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