As Haakon I's reign stretched into the 960s he faced increasing challenges to his rule. His attempts to impose Christianity were largely failing, his wars against the rulers of Viken; Olaf I and Tryggve were damaging what authority he had in the centre and he was constantly under threat of invasion by the sons of Eirik Bloodaxe.
These sons; Guttorm, Gamle, Harald Gráfledr (Greycloak) and Sigurd, frequently chanced their luck with invasions from their exile in Denmark. As their uncle was Harald Bluetooth, King of Denmark they could often rely on Danish assistance in their attempts. Haakon however was a good commander and saw off their attempts with skillful use of what little army he could muster. Guttorm invaded in 953, and was killed on the battlefield. The younger three struck in 955 and 957. It would be Harald's invasion of 961 which finally succeeded. Although Haakon technically won the Battle of Fitjar he was mortally wounded in the process and when he died the Hordalandic nobles elected Harald as their new king.
Although he initially bowed to Harald Bluetooth's overlordship Harald was ambitious. He successfully limited the power of the semi-independent Earldom of Lade, mainly by killing its Earl, Sigurd Haakonsson, and extended his authority to the North into Hålogaland. Meanwhile voyages around the top of Scandinavia brought Norse influence to what would eventually become the northern ports of Novgorod.
In 963 he achieved what his father could never do: conquer Viken. He soon had no need of assistance from Denmark and effectively threw off their overlordship. However he had made many enemies in his ruthless quest for power. In 970 the son of Sigurd Haakonsson, Haakon Sigurdsson, himself a Danish ally, tricked Harald into coming to Denmark. There Harald and his men were set upon and killed. Harald Bluetooth divided Norway once more, taking Viken for himself. Hordaland he entrusted to Haakon Sigurdsson.