Adolf Menzel, Coronation of Wilhelm I in Königsberg in the Year 1861 (1861) Having ruled Prussia since 1858 on behalf of his mentally ill brother, Friedrich Wilhelm IV (1795-1861), Wilhelm I became king on October 18, 1861. With the help of Otto von Bismarck, he proceeded to institute a more authoritarian regime, despite resistance from the liberal majority in parliament. Painting (1861) by Adolf Menzel.