In September 1688, King Louis XIV's forces invaded the Rhineland and besieged the German town of Philippsburg. Louis had hoped to compel the powerful German princes, and their Holy Roman Emperor, Leopold I, into converting the 1684 Truce of Ratisbon into a permanent peace, thus confirming Louis' territorial gains of the ‘Reunions’. Other German towns fell in quick succession, including Oppenheim, Kaiserslautern, Heidelberg, and the key fortress of Mainz, but instead of cowering under French aggression, the German princes united against Louis' forces. What was supposed to be a campaign lasting only months, evolved into the Nine Years' War.