Simplify the following text for a Grade 13 native English-speaking person.
Original: The monument commemorates the Cherusci war chief Hermann or Armin ( Latin : Arminius ) and the Battle of the Teutoburg Forest in which the Germanic tribes under Arminius recorded a decisive victory in 9 AD over three Roman legions under Varus .
Simplified: The monument commemorates the Cherusci war chief Hermann or Armin ( Latin : Arminius ) and the Battle of the Teutoburg Forest in 9 AD when the Germanic tribes under Arminius fought three Roman legions and other troops under Varus .