By about 400 CE, the Romans abandoned Britain. Their legions were needed to help defend the Roman heartland and Britain had always been an imperial frontier, with too few Romans to completely settle and “civilize” it outside of southern England. For the next three hundred years, Germanic invaders called the Anglo-Saxons (from whom we get the name “England” itself - it means “land of the Angles”) from the areas around present-day northern Germany and Denmark invaded, raided, and settled in England. They fought the native Britons (i.e. the Romanized, Christian Celts native to England itself), the Cornish, the Welsh, and each other. Those Romans who had settled in England were pushed out, either fleeing to take refuge in Wales or across the channel to Brittany in northern France. England was thus the most thoroughly de-Romanized of the old Roman provinces in the west: Roman culture all but vanished, and thus English history “began” as that of the Anglo-Saxons.
Starting in the late eighth century, the Anglo-Saxons suffered waves of Viking raids that culminated in the establishment of an actual Viking kingdom in what had been Anglo-Saxon territory in eastern England. It took until 879 for the surviving English kingdom, Wessex, to defeat the Viking invaders. For a few hundred years, there was an Anglo-Saxon kingdom in England that promoted learning and culture, producing an extensive literature in Old English (the best preserved example of which is the epic poem Beowulf). Raids started up again, however, and in 1066 William the Conqueror, a Viking-descended king from Normandy in northern France, invaded and defeated the Anglo-Saxon king and instituted Norman rule.