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.