I am particularly interested in the defeat of the Legions of Varus 9 A.D. in the Teutoburger forest by Arminius of the Cheruski. In 17 A.D. (?) Germanicus went for payback but faced an even greater defeat. It resulted in the destruction of almost every active(!) legion in Europe. Augustus and Tiberius had to take serious measures in order to vauchsafe the protection of the Roman Empire.
It is fascinating how one man almost brought the mighty empire to its knees.