He gained a certain amount of theoretical support from some of the German princes after his marriage to Elizabeth, daughter of the Welf duke Otto of Brunswick-Lüneburg, in 1252; but, although "William lacked neither courage nor chivalrous qualities... his power never extended beyond the Rhineland."
Han fik en vis mængde teoretisk støtte fra nogle af de tyske fyrster efter hans ægteskab med Elisabeth, datter af Otto Barnet, hertug af ​​Braunschweig-Lüneburg, i 1252; men, selv om "Vilhelm manglede hverken mod eller ridderlige kvaliteter ... nåede hans magt aldrig ud over Rheinland."