In 1701 the Holy Roman Emperor and Polish King allowed Brandenburg-Prussia to have the name `` Kingdom of Prussia '' under Frederick I. Under Frederick II ( `` the Great '' ) , Prussia took Silesia from Austria and kept it in the Silesian Wars . On 18 January 1701 , Frederick William 's son , Elector Frederick III , upgraded Prussia from a duchy to a kingdom and crowned himself King Frederick I. To avoid offending Poland , where a part of the old Prussia lay , Leopold I , emperor of the Holy Roman Empire where most of the lands of Prussia lay , allowed Frederick only to title himself `` King in Prussia '' , not `` King of Prussia '' .