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When Magnus Ladulås died in 1290 his son Birger Magnusson was only 10 years. He had already been elected as successor as a four-year-old, and he (and the kingdom of Sweden) now had a guardian, marshal Torgils Knutsson. In 1304 the dukes signed a document that forbade them from conducting their own foreign policy and kept them out of the royal court unless they had specifically been summoned. The bitterness over this agreement was probably the cause of the Håtuna games.
When marshal Torgils Knutsson returned from the third and last crusade in Finland in 1293 a feud had developed between the brothers, with the marshal supporting King Birger. Duke Eric, as a leader of men and politician possibly better king material than Birger, tried to establish an independent kingdom around Bohuslän, which he had received as part of his marriage to the princess Ingeborg of Norway, and Halland at the boundary between Sweden, Norway and Denmark. A civil war broke out, but by 1306 emotions had cooled to the point where the dukes acknowledged the son of Birger, Magnus Birgersson, as the successor to the throne.
The Håtuna games
On September 29, 1306, Duke Eric and Duke Valdemar arrived at king Birger's estate Håtuna by Lake Mälaren. They came from a wedding feast in Bjälbo, and Birger received them as guests at a party that derailed towards in the middle of the night. Birger and his wife Martha of Denmark were captured by the dukes, and were imprisoned in the dungeon in Nyköping Castle. King Birger's son Magnus Birgersson was rescued by a courtier and sent to Denmark where he was taken up by king Eric VI.
Duke Eric took over power in Sweden. He was already sovereign in Halland and Bohuslän and, being married to princess Ingeborg, had aspirations to inherit the crown of Norway. But the closeness to Denmark became a threat, and King Eric VI of Denmark attacked Sweden, harassing Västergötland. Duke Eric's brother Valdemar wreaked revenge by plundering Skåne with a force of German mercenaries. The Norwegian king allied himself with the Danish king; this forced a settlement that allowed Birger to leave Nyköping Castle after two years' imprisonment.
During 10 years the brothers kept the peace with only smaller skirmishes. In 1317-1318 the Håtuna games had their final resolution at the gruesome Nyköping Banquet and subsequent events.