Initially a vassal of the dukes of Saxony, Oldenburg became a county directly suzerain to the Holy Roman Emperor in the 12th century. In 1450, when count Christian VI became king Christian I of Denmark, the country entered into a personal union with Denmark. A second union with Denmark existed from 1667 to 1773. Oldenburg's dynasty also ruled Schleswig-Holstein, Russia and Sweden at some point in time. Oldenburg became a grand duchy in 1815 and a republic in 1918. In 1931 it was the first German state to vote the Nazi Party into power. After World War II, Oldenburg was merged with Hanover to form the state of Lower Saxony.