For Athelstan it was very important to have many allies. A year after his crowning he married one of his sisters to the Viking King of Northumbria, Sithric. However, Sithric died only a year later, and Athelstan seized Northumbria. This move made him king of more land than any other before him, roughly the same as modern England. Historical records indicate that the other rulers in Great Britain accepted him as king at Bamburgh around this time.