The two were first united by king Æthelfrith around the year 604, and except for occasional periods of division over the subsequent century, they remained so.
After the Mercian defeat at Winwaed, Œthelwald lost power and Oswiu's own son, Alchfrith, became king in his place.
Ælfwine was killed in battle against Mercia in 679, and there was not another separate king of Deira until the time of Norse rule.
Viking kings ruled Jórvík (southern Northumbria, the former Deira) from its capital York for most of the period between 867 and 954.
After the ascension of William the Conqueror in 1066, and in the prelude to the Harrying of the North, a joint Anglo-Danish force loyal to Edgar Ætheling and Sweyn II was able to seize control of York in the second half of 1069 and temporarily assert sovereignty over all of Northumbria.