Original: Before 1866 , its northern neighbor was its former sister Landgraviate , since 1803 an Electorate , of Hessen-Kassel -- for this reason , this state was sometimes colloquially known as Hesse-Darmstadt .
Simplified: Because Hesse-Darmstadt was a member of Napoleons Confederation of the Rhine , it was forced to give up a considerable amount of territory at the 1815 Congress of Vienna .