Duke Haakon, the second son of Magnus Law-Mender, succeeded his brother without opposition. He was then twenty-nine years old, tall and of stately appearance. He had not been long upon the throne, before he showed the haughty barons that he meant to have a reckoning with them. He first summoned Sir Audun Hugleiksson to meet him in Bergen, tried him for treason, and had him executed (1302). A woman from LÜbeck had, two years before, appeared in Norway and created much excitement by claiming to be the Princess Margaret, "The Maid of Norway," who had died on the Orkneys. Her trial proved her to be an impostor, and she was burned at the stake. According to one conjecture, Sir Audun was in some way compromised by her trial, and it is not unlikely that he may have encouraged her pretensions. The legend, however, relates that Sir Audun suffered death for having insulted the king's bride, Countess Euphemia of Arnstein, whom, in 1295, he brought over from Germany.

It must have been an unpleasant surprise to the barons, who had had their own way so long, to find a stern and determined master in the new king, and it is the more to his credit that, in spite of their hostility, he induced them to consent to a change in the law of succession in favor of his daughter Ingeborg and her issue. As he was the only male descendant in the direct line of the old royal house, it was a source of uneasiness to him that he had no sons, and he foresaw that the only means of averting civil war, after his death, was to secure the succession to the prospective sons of his daughter, and in case she had none, to herself. Princess Ingeborg was, while a mere child, promised in marriage to the brilliant and ambitious Duke Erik, the second son of King Magnus Birgersson of Sweden. By this betrothal, King Haakon became involved in the quarrels of the dukes Erik and Valdemar with their brother, Birger Magnusson, whom they were endeavoring to dethrone. The dukes hated the king, and the king, who was jealous of Erik's popularity and eminence in chivalrous accomplishments, reciprocated their feelings. The long-smouldering hostility at last blazed forth, in 1306, when the dukes treacherously assaulted their brother and held him captive for about eighteen months. King Haakon was induced to take their part in the struggle, perhaps chiefly because his enemy, the king of Denmark, made common cause with King Birger. The good understanding between them did not, however, last long, for when it began to look as if Duke Erik aimed at the union of the three Scandinavian kingdoms under his own sceptre, Haakon, as an interested party, could scarcely remain inactive. He demanded the restoration of the fiefs which he had granted the duke during his exile. When this was refused, he opened negotiations with the king of Denmark, who was the brother-in-law of King Birger, and concluded a preliminary treaty at Copenhagen (1308) in accordance with which the Princess Ingeborg was to marry Magnus, the son of King Birger. Duke Erik then invaded Norway with an army, took Oslo and vainly besieged the fortress of Akershus. The province of Jemteland was also attacked by the Swedes, and the duke had in 1309 an indecisive fight with a portion of the Norwegian fleet in Kalfsund. Finally, after another fight, in which Erik gained the upper hand, negotiations were resumed, and by mutual concessions peace was reËstablished (1310). Duke Erik had a powerful ally at the Norwegian court in Queen Euphemia, whose love for him was not of an entirely maternal character. He had thus little difficulty in conciliating King Haakon and getting again the promise of his daughter's hand. The wedding was finally celebrated with much splendor in Oslo in 1312. Duke Valdemar married the same day the king's niece, Ingeborg, the daughter of King Erik Priest-Hater. About four years later, when the hope had almost been abandoned, each of the duchesses bore a son. King Haakon's joy at this happy event was great, for it relieved him of his anxiety for the succession. But his joy was of short duration. There was one man in Sweden who was not rejoiced at the birth of the young princes, and that was King Birger. He feigned, however, delight, and invited his brothers to a great feast of reconciliation at the castle of NykÖping. When the festivities were at an end, the dukes were thrown into prison and deprived of their lives. As there was no sign of violence on their bodies, the rumor went abroad that they had been starved to death. This was probably true. The tidings of this calamity gave King Haakon such a shock that he never recovered from it. He died, 1319, aged forty-nine years. With him the male line of the race of Harold the Fairhaired became extinct.

The war with Denmark which had lasted twenty-eight years, was continued in a desultory fashion during Haakon's reign, but no important battles were fought. He used his fleet mainly as a threat to enforce his claims. All that he gained was the temporary possession of the province of Northern Halland, as security for the final surrender of his maternal inheritance.

In internal affairs King Haakon exhibited, according to the ideas of his age, no mean degree of statesmanship. His administration was both prudent and vigorous. He checked the usurpations of the Hanseatic cities, which were driving native merchants out of the foreign trade, and deprived them of some of their privileges. An honest intention to do right, coupled with considerable ability, characterized both his public and private life. For all that, his despotic temper tended to alienate the people from public affairs; and thus prepared the way for the following centuries of humiliation.