Magnus was proclaimed king in 1035 at 11 years of age while Svein, the pretender to the Norwegian throne as son of Canute the Great of Denmark and Queen Elfgifu of Mercia, fled; Svein died shortly after.
At first Magnus sought revenge against his father's enemies, but on Sigvatr's advice he stopped doing so, which is why he became known as "good" or "noble".

Another son of Cnut, Harthacnut, was on the throne of Denmark and wanted his country to reunite with Norway, while Magnus initiated a campaign against Denmark around 1040.
However, the noblemen of both countries brought the two kings together at the Göta älv, the border between their kingdoms.
They made peace and agreed that the first of them to die would be succeeded by the other.
In 1042 Harthacnut died while in England, and Magnus also became King of Denmark, in spite of a claim by Cnut's nephew Sweyn Estridsen, whom Harthacnut had left in control of Denmark when he went to England, and who had some support.

As part of consolidating his control, Magnus destroyed the Jomsborg, headquarters of the Jomsvikings.
Sweyn fled east and returned as one of the leaders of an invasion by the Wends in 1043, which Magnus decisively defeated at the Battle of Lyrskov Heath, near Hedeby.
In the battle, Magnus wielded Saint Olaf's battle-axe, named Hel after the goddess of death.
He had dreamt of his father the night before, and the Norwegians swore that before the battle they could hear the bell that Saint Olaf had given to the Church of St. Clement in Kaupang, in Nidaros – a sign that the saint was watching over his son and the army.
It was the greatest victory ever over the Wends, with up to 15,000 killed.

Sweyn continued to oppose Magnus in Denmark, although according to Heimskringla, they reached a settlement by which Sweyn became Earl of Denmark under Magnus.

Magnus wanted to reunite Cnut the Great's entire North Sea Empire by also becoming king of England.
When Harthacnut died, the English nobles had chosen as their king Æthelred the Unready's son Edward (later known as Edward the Confessor); Magnus wrote to him that he intended to attack England with combined Norwegian and Danish forces and "he will then govern it who wins the victory."
 The English were mostly hostile to Magnus; Sweyn was made welcome there, although Edward's mother, Emma, curiously favored Magnus and in 1043 the king confiscated her property, with which by one report she had promised to assist Magnus.

Meanwhile, Magnus' uncle Harald Sigurdsson had returned to Norway from the east and contested his rule there, while Sweyn was still a threat in Denmark; Harald allied himself with Sweyn.
Magnus chose to appease Harald, and made him his co-king in Norway in 1046.
Magnus was proclaimed king in 1035 at 11 years of age while Svein, the pretender to the Norwegian throne as son of Canute the Great of Denmark and Queen Elfgifu of Mercia, fled; Svein died shortly after. At first Magnus sought revenge against his father's enemies, but on Sigvatr's advice he stopped doing so, which is why he became known as "good" or "noble".
Another son of Cnut, Harthacnut, was on the throne of Denmark and wanted his country to reunite with Norway, while Magnus initiated a campaign against Denmark around 1040. However, the noblemen of both countries brought the two kings together at the Göta älv, the border between their kingdoms. They made peace and agreed that the first of them to die would be succeeded by the other. In 1042 Harthacnut died while in England, and Magnus also became King of Denmark, in spite of a claim by Cnut's nephew Sweyn Estridsen, whom Harthacnut had left in control of Denmark when he went to England, and who had some support.
As part of consolidating his control, Magnus destroyed the Jomsborg, headquarters of the Jomsvikings. Sweyn fled east and returned as one of the leaders of an invasion by the Wends in 1043, which Magnus decisively defeated at the Battle of Lyrskov Heath, near Hedeby. In the battle, Magnus wielded Saint Olaf's battle-axe, named Hel after the goddess of death. He had dreamt of his father the night before, and the Norwegians swore that before the battle they could hear the bell that Saint Olaf had given to the Church of St. Clement in Kaupang, in Nidaros – a sign that the saint was watching over his son and the army. It was the greatest victory ever over the Wends, with up to 15,000 killed.
Sweyn continued to oppose Magnus in Denmark, although according to Heimskringla, they reached a settlement by which Sweyn became Earl of Denmark under Magnus.
Magnus wanted to reunite Cnut the Great's entire North Sea Empire by also becoming king of England. When Harthacnut died, the English nobles had chosen as their king Æthelred the Unready's son Edward (later known as Edward the Confessor); Magnus wrote to him that he intended to attack England with combined Norwegian and Danish forces and "he will then govern it who wins the victory."  The English were mostly hostile to Magnus; Sweyn was made welcome there, although Edward's mother, Emma, curiously favored Magnus and in 1043 the king confiscated her property, with which by one report she had promised to assist Magnus.
Meanwhile, Magnus' uncle Harald Sigurdsson had returned to Norway from the east and contested his rule there, while Sweyn was still a threat in Denmark; Harald allied himself with Sweyn. Magnus chose to appease Harald, and made him his co-king in Norway in 1046.
Magnus the Good