Northumbria was originally composed of the union of two independent kingdoms, Bernicia and Deira. It stretched from the Humber to the Firth of Forth and included Edinburgh as one of its cities. The northern half of the kingdom was ceded to the new kingdom of Scotland after a Northumbrian defeat in the 9th century. The links between Northumbria and Scotland are clearly visible today in the dialects of both, which includes many Old English words, such as 'Bairn' for 'Child'. For further information, see Scots language and Geordie.

Beginnings

Bernicia and Deira were first united by Aethelfrith, a king of Bernicia who conquered Deira around the year 604. He was defeated and killed around the year 616 in battle at the River Idle by Raedwald of East Anglia, who installed Edwin, the son of Aella, a former king of Deira, as king. Edwin, who accepted Christianity in 627, soon grew to become the most powerful king in England: he was recognized as Bretwalda and conquered the Isle of Man and Gwynedd in northern Wales. He was, however, himself defeated by an alliance of the exiled king of Gwynedd, Cadwallon ap Cadfan and Penda, king of Mercia, at the Battle of Hatfield Chase in 633.

After Edwin's death, Northumbria was split between Bernicia, where Eanfrith, a son of Aethelfrith, took power, and Deira, where a cousin of Edwin, Osric, became king. Both of these rulers were killed during the year that followed, as Cadwallon continued his devastating invasion of Northumbria. After the murder of Eanfrith, his brother, Oswald, backed by a force of Scots sent by Domnal of Dalriada, defeated and killed Cadwallon at the Battle of Heavenfield in 634.

King Oswald

King Oswald re-introduced Christianity to the Kingdom, but this time, by appointing St Aidan, an Irish monk from the Scottish island of Iona to convert his people. This led to the introduction of Celtic Christianity, as opposed to Roman Catholicism. A monastery was established on Lindisfarne, probably as an echo of the island monastery of Iona.

War with Mercia continued, however. In 642, Oswald was killed by the Mercians under Penda at the Battle of Maserfield. In 655, Penda launched a massive invasion of Northumbria, aided by the sub-king of Deira, Aethelwald, but suffered a crushing defeat at the hands of an inferior force under Oswiu, Oswald's successor, at the Battle of Winwaed. This battle marked a major turning point in Northumbrian fortunes: Penda died in the battle, and Oswiu gained supremacy over Mercia, making himself the most powerful king in England.

Religious Union and the Loss of Mercia

In the year 664 a great synod was held at Whitby to discuss the controversy regarding the timing of the Easter festival. Much dispute had arisen between the practices of the Celtic church in Northumbria and the beliefs of the Roman church. Eventually, Northumbria was persuaded to move to the Roman faith, the Celtic Bishop Colman of Lindisfarne returned to Iona.

Northumbria lost control of Mercia in the late 650s, after a successful revolt under Penda's son Wulfhere, but it retained its dominant position until it suffered a disastrous defeat at the hands of the Picts at the Battle of Nechtansmere in 685; Northumbria's king, Ecgfrith (son of Oswiu), was killed, and its power in the north was gravely weakened. The peaceful reign of Aldfrith, Ecgfrith's half-brother and successor, did something to limit the damage done, but it is from this point that Northumbria's power began to decline, and chronic instability followed Aldfrith's death in 704.

See also: List of monarchs of Northumbria

More detail needed to take this past St. Cuthbert, the Lindisfarne Gospels, Norman Conquest and eventually into the present day.