How did Northumbria become a permanent part of the kingdom of England?

Northumbria became a permanent part of the kingdom of England following the collapse of Scandinavian power in the region during the 10th century.12

While Northumbria had been a powerful Anglo-Saxon kingdom, its political unity was shattered by the arrival of the Danish "great army" in 866. Throughout the early 10th century, the region was contested by various Scandinavian leaders, particularly those based in Ireland. Although Athelstan briefly held direct rule over Northumbria starting in 927, control remained unstable after his death in 939, with the territory frequently changing hands between West Saxon kings and Norse rulers like Erik Bloodax.123

The transition to a permanent part of the English kingdom was finalized in 954, when Erik Bloodax was killed. Following this event, independent kings of Northumbria ceased to exist, and the region was integrated into the kingdom of England as an earldom.31