Northumbria, kingdom of (nôrthŭmˈbrēˈə) [key], one of the Anglo-Saxon kingdoms in England. It was originally composed of two independent kingdoms divided by the Tees River, Bernicia (including modern E Scotland, Berwick, Roxburgh, E Northumberland, and Durham) and Deira (including the North and East Ridings of Yorkshire), both settled by invading Angles c.500. Sparse records tell of a King Ida of Bernicia and a King Ælli or Ælle of Deira in the middle of the 6th cent. Æthelfrith of Bernicia (593–616) united the kingdoms to form Northumbria and added Scottish and Welsh territory. He was defeated by Edwin of Deira (616–32), who accepted (627) Roman Christianity and established Northumbrian supremacy in England. Edwin was killed by Cadwallon of the Welsh kingdom of Gwynned, an ally of Penda of Mercia. After a year of anarchy he was succeeded by Oswald of Bernicia (633–41), who brought in St. Aidan to introduce Celtic Christianity. Oswald was killed by Penda. Under Oswald's successors, Osiu (641–70) and Ecgfrith (670–85), Northumbria's power gradually declined as that of Mercia increased. Osiu, however, established the Roman Church over the Celtic Church at the Synod of Whitby (663). The late 7th and 8th cent. saw almost constant political discord during the golden age of the Church, arts, scholarship, and literature in Northumbria. The Danes invaded with their victory at York in 867. They occupied S Northumbria, and the Angles were able to keep only a small kingdom stretching from the Tees N to the Firth of Forth. The conquering Canute (1015) and his successors installed Danish earls, of whom Siward (d. 1055) was the last and most powerful. The Northumbrians expelled his successor, Tostig, in 1065. He was replaced by Morcar, the brother of Edwin, earl of Mercia. The next year Tostig returned with Harold Hardrada of Norway and defeated Morcar and Edwin at Fulford. Harold II of England, however, soon came north to defeat the Danes.