The island was governed as an independent commonwealth under the Althing, one of the world's oldest functioning legislative assemblies. Following a period of civil strife, Iceland acceded to Norwegian rule in the 13th century. The establishment of the Kalmar Union in 1397 united the kingdoms of Norway, Denmark, and Sweden. Iceland thus followed Norway's integration into that union, coming under Danish rule after Sweden's secession from the union in 1523. Although the Danish kingdom introduced Lutheranism forcefully in 1550, Iceland remained a distant semi-colonial territory in which Danish institutions and infrastructures were conspicuous by their absence.