Main article: Sub-Roman Britain
In the wake of the breakdown of Roman rule in Britain from the middle of the fourth century, present day England was progressively settled by Germanic groups. Collectively known as the "Anglo-Saxons", these were Angles and Saxons from, what is now, the Danish/German border area and Jutes from the Jutland peninsula. The entire region was referred to as, "Hwicce" and settlements throughout the south were called Gewisse. The Battle of Deorham was a critical battle that established the Anglo-Saxon rule in 577. Saxon mercenaries had been present in Britain since before the late Roman period, but the main influx of population is thought to have taken place after the fifth century. The precise nature of these invasions has not been fully determined, with doubts being cast on the legitimacy of historical accounts due to a lack of archaeological finds. The Monarchs of Mercia's lineage was determined to reach as far back as the early 500's.
Seven Kingdoms are traditionally identified as being established by these Saxon migrants. Three were clustered in the South east: Sussex, Kent and Essex. The Midlands were dominated by the kingdoms of Mercia and East Anglia. To the north was Northumbria which unified two earlier kingdoms, Bernicia and Deira. The development of these kingdoms led to the eventual domination by Northumbria and Mercia in the 7th century, Mercia in the 8th century and then Wessex in the 9th century. Northumbria extended its control north into Scotland and west into Wales. It also subdued Mercia whose first powerful King, Penda, was killed by Oswy in 655. Northumbria's power began to wane after 685 with the defeat and death of its king Aegfrith at the hands of the Picts. Mercian power reached its peak under the rule of Offa, who from 785 had influence over most of Anglo-Saxon England. From Offa's death in 796 the supremacy of Wessex was established under Egbert who extended his control west into Cornwall before defeating the Mercians at the Battle of Ellendun in 825. Four years later he received submission and tribute from the Northumbrian king, Eanred.
The sequence of events of the fifth and sixth centuries is particularly difficult to access, peppered with a mixture of mythology, such as the characters of Hengist and Horsa, and legend, such as St Germanus's so-called "Alleluia Victory" against the Heathens, and half-remembered history, such as the exploits of Ambrosius Aurelianus and King Arthur. However, the belief that the Saxons simply wiped or drove out all the native Britons from England has been widely discredited by a number of archaeologists since the 2000s, and the likelihood of that model being severely questioned. At any rate, the Anglo-Saxons, including Saxonified Britons, progressively spread into England, by a combination of military conquest and cultural assimilation, until by the eighth century some kind of England really had emerged.