Thus the political historian sees the ninth century as the crucial period in the evolution of a unitary English kingdom. The four powerful and independent Anglo-Saxon kingdoms existing in 800—Northumbria, Mercia, East Anglia and Wessex—had been reduced by the end of the century to one, Wessex.

England in the ninth century. A large-scale map of southern England (up to the Humber), showing the then settlements and political divisions, including the line of the Alfred-Guthrum treaty. (Earlier kingdoms are yielding importance to the line between "English" Mercia and Wessex and the eastern half of the country under Viking control).