Haakon Ivarson showed great zeal in his position as commander of the Danish coast defence, being out with his warships both winter and summer, and was in high favor with King Svein, until he attacked and killed the king's nephew, Asmund, an ungovernable young man, who had been killing and plundering everywhere, both abroad and at home, and whom Haakon thought the king much desired to get rid of. The king sent Haakon a message that he had better leave the country. "Tell him," he said, "that I will do him no harm; but I cannot answer for all our relations." Haakon then proceeded north to his estates in Norway. During his stay in Denmark his relative, Earl Orm, had died. His many friends therefore gave themselves much trouble to bring about a reconciliation between him and King Harald, and in this they succeeded. Haakon was given the title of earl, with the same power that Earl Orm had had, and was married to Ragnhild, King Magnus's daughter. He swore to King Harald an oath of fidelity and pledged himself to render all the service he was liable to.
In the winter of 1061-62, King Harald resided at Nidaros, where he commenced building a large warship. He sent a message south to Denmark to King Svein, and challenged him to meet him in the spring at the Gaut River and fight, with the understanding that the one who gained the victory should have both kingdoms. King Svein accepted the challenge, but did not keep the appointment. King Harald, who had arrived at the place agreed upon, heard that Svein's forces lay in the south, partly at Fyen and partly about Seeland. Harald then sailed southward along Halland with one hundred and eighty of his ships, and brought up his fleet at the Nis-Aa (Nis River). Shortly afterward King Svein came upon them with a Danish fleet consisting of three hundred and sixty ships. King Harald held a war council, and many said that it would be better to fly than to fight with a fleet twice the size of their own. The king replied: "Sooner shall we all fall and lie dead one upon another than fly." King Harald drew up his ships in battle array, laying his great dragon ship in the middle. At his side lay Ulf Stallara, and on the other wing lay the ships of Earl Haakon Ivarson from the Uplands. At the extremity of the other side lay the Throndhjem chiefs. It was late in the day when the battle began, and it continued the whole night. The battle was very severe, and toward morning the greater part of the Danish fleet broke into flight. While Harald pursued some of the Danes, King Svein made his escape with the aid of Earl Haakon Ivarson, who, during the battle, had contributed more than any one else to the victory of the Norwegians.
