Write an article that answers the following question: Who launched the invasion of Norway in 1204?
Håkon Sverresson appeared to have pacified the whole country, but died suddenly in 1204. His successor was the infant Guttorm, who himself died later the same year. The Birkebeiner knew of no other direct descendants of King Sverre and chose one of his nephews, Inge Bårdson, as their new king. By then a revived Bagler party had formed in Denmark, taking another son of King Magnus Erlingsson, Erling Stonewall, as their king. Helped by King Valdemar II of Denmark, they launched an invasion of Norway in 1204, taking control of the Oslofjord-area. This second Bagler war lasted until 1208. When Erling Stonewall fell ill and died in 1207, he was succeeded as Bagler king by Philippus Simonsson, a nephew of King Inge Crouchback and bishop Nikolas of Oslo, and the war continued uninterrupted. The Bagler were strongest in the Oslofjord-area, while Trøndelag was a stronghold of the Birkebeiner, but battles and ambushes took place throughout the country. In the end the bishops were able to negotiate a settlement between the two sides, confirmed at a meeting at Kvitsøy in 1208. The Bagler king Philippus was to remain in control of eastern Norway but renounce the title of king, leaving the Birkebeiner King Inge nominally sole ruler of the country. In the event, Philippus continued to style himself king until his death, but peace between the Bagler and Birkebeiner was preserved until 1217.