In early 1365, Haakon and Magnus assembled a large army in Västergötland, consisting mainly of Norwegians, but also a large number of Swedes from the said province, and marched on the German-held city of Stockholm. On 27 February, Haakon issued a proclamation against Albert of Mecklenburg and his supporters, encouraging the local populace to stir up in rebellion against the German usurper. The Norwegian army entered Uppland through Västerås and clashed against the Swedish-German army in the disastrous Battle of Gataskogen where Haakon and Magnus suffered a devastating defeat and Magnus was captured and taken prisoner by the Germans; which he would remain for six years.