so basically what happened was Arminius was the son of a chieftan of one of the Germanic tribes, he was a hostage as a kid who grew up in Rome and learned their tactics and became a military commander, 

he faked loyalty and setup 3 roman legions to march through a narrow path in the Teutoburg forest, where the Germanic tribes of the area would then ambush them so that they could pick them off and the Romans could do nothing about it, and the romans lost over 20k soldiers which devastated them so yeah