Eric is reported as being more skilled and intelligent than his elder brother who became King Birger of Sweden. He was also bold and ambitious, and his social skills won him many allies. His younger brother Valdemar Magnusson, the duke of Finland, became his close ally and helped him in all his projects.
King Birger, who feared his brothers' plans, forced them to sign a paper, in 1304, so as to render them less dangerous. They then fled to Norway, but in 1305, they reconciled with the king and regained their duchies.
Eric was also in possession of Kungahälla, which he had been given during his exile by the Norwegian king, and northern Halland which he had been given by the Danish king Eric VI of Denmark. Duke Eric planned to topple Birger's marshal Torgils Knutsson who was in the way of his ambitious plans. As the clergy were in opposition to the marshal, they joined Eric. They prevailed on the weak Birger in 1306 to execute Torgils, who was a faithful counsellor. Little more than half a year later, Birger was imprisoned by his brothers (September 1306), and his brothers took control of Sweden.
Birger's brother-in-law, Eric VI of Denmark arrived with his army to support Birger. Haakon V of Norway, however, was on the side of the younger brothers. In 1308, Eric and Valdemar were forced by the Danish king to release Birger, but they did so under humiliating conditions. When Birger was free, he sought aid in Denmark, and the strife began anew. The course of events turned against duke Eric. By concluding a peace treaty with the Danish king, unbeknownst to Haakon V, Eric lost Haakon's trust. Håkon wanted to have Kungahälla back, but Eric refused.