The Treaty of Kalmar was signed on 25 September 1397 between representatives of the three Scandinavian kingdoms of Sweden, Denmark, and Norway. The accord established the Kalmar Union whereby all three realms were to be ruled by one monarch. However, the treaty did not unite the different legal structures of each kingdom. The agreement was broken when Gustav I of Sweden left the Kalmar Union on 6 June 1523.
References
Treaty of Kalmar Wikipedia(Text) CC BY-SA