Following the Roman retreat, Britain was left open to invasion by pagan, seafaring warriors such as Saxons and Jutes who gained control in areas around the south-east.[29] This advance was contained for a while after the Britons led the Battle of Mount Badon. In the north there were Sub-Roman Brythonic kingdoms known collectively as the Hen Ogledd, however these too were gradually conquered by Angles during the 6th century. Reliable contemporary accounts from this period are scarce, as is archaeological evidence, meaning it was a Dark Age. There are various conflicting theories on the extent and process of the Anglo-Saxon invasion of Britain—to conflate the issue Cerdic, founder of the Wessex dynasty may have been a Briton.[30] Nevertheless, by the 7th century a coherent set of Anglo-Saxon petty kingdoms known as the Heptarchy had emerged in southern and central Britain: Northumbria, Mercia, East Anglia, Essex, Kent, Sussex, and Wessex.[31] Christianity was reintroduced in the south by Augustine from Rome and in the north by Aidan from Ireland.[32] The title Bretwalda meaning Lord of the Britons denoted the most influential kingdom.[33] Northumbria and Mercia were the most dominant forces early on. However, following Viking conquests in the north and east, with the insertion of Danelaw, the premier English kingdom became Wessex under Alfred the Great. His grandson Athelstan unified England in 927, though this was only made permanent when Edred defeated Eric Bloodaxe. A short-lived empire under Viking king Cnut the Great which included England, Denmark and Norway existed.[34] However the Wessex dynasty was restored under Edward the Confessor.
Its ties and nature, however, were forever changed following the Norman conquest of England in 1066.[why?] In the next few hundred years, English kings controlled parts of France, and the "Kings of England" used England as a source of troops to enlarge their personal holdings in France (Hundred Years' War). The English crown did not relinquish its last foothold on mainland France until Calais was lost, in 1558, during the reign of Mary Tudor (the Channel Islands are still crown dependencies, though not part of the UK). In the 13th century Wales (the remaining Romano-Celts) was brought under the control of English monarchs by conquest. This was formalised in the Statute of Rhuddlan in 1284 and Wales was legally annexed to the Kingdom of England by the Laws in Wales Acts 1535–1542. Wales shared a legal identity with England as the joint entity originally called England, and later England and Wales. The Black Death, an epidemic of catastrophic proportions, first reached England in the summer of 1348. The Black Death is estimated to have killed between a third and two-thirds of Europe's population. England alone lost between a third to half of its inhabitants.[35][36] The plague repeatedly returned to haunt England throughout the 14th to 17th centuries. The Great Plague of London in 1665–1666 was the last such plague outbreak.
