Sweyn Estrithson, born in England as the son of Norwegian Ulf Jarl and Estrid Svendsdatter, the daughter of King Sweyn I of Denmark and sister of king Cnut the Great, had grown up a military leader, and had served under king Anund Jacob of Sweden for a time.

He had pillaged the Elbe-Weser area in 1040, but had been caught by the Archbishop of Hamburg-Bremen, who released him shortly thereafter.

Sweyn had been made jarl under Danish king Harthacnut, and had led a campaign for him against Norway, but had been beaten by Magnus I of Norway.

When Harthacnut died in 1042, Magnus had claimed the Danish throne and made Sveyn his jarl of Jutland.

Sweyn had fought for Magnus in 1043 at the Battle of Lyrskov Heath.

Sweyn had won a great reputation at Lyrskov Heath, and had had the Danish nobles crown him king in Viborg in Jutland.

Defeated by Magnus on several occasions, he had had to flee to Sweden.

Eventually he managed to return and establish a foothold in Scania.

The war between Magnus and Sweyn had lasted until 1045, when Magnus' uncle Harald Hardrada returned to Norway from exile.

Harald and Sweyn had joined forces, and Magnus had decided to share the Norwegian throne with Harald.

Magnus dies in 1047, having stated on his deathbed that his kingdom is to be divided: Harald will get the throne of Norway, while Sweyn will be king of Denmark.

Harald, unwilling to relinquish Denmark, attacks Sweyn, initiating what will be a long and brutal war.