Simplify the following text for a Grade 11 native English-speaking person.
Original: Due to Hesse-Darmstadt 's membership of Napoleon 's Confederation of the Rhine , it was forced to cede a considerable amount of territory at the 1815 Congress of Vienna .
Simplified: Because Hesse-Darmstadt was a member of Napoleon 's Confederation of the Rhine , it was forced to give up a considerable amount of territory at the 1815 Congress of Vienna .