This video details the event known as the “Teutoburg forest massacre” where three entire Roman legions (about 20,000 Roman soliders) were led into a trap and massacred in the Teutoburg Forest in Germany in 9 AD, being the largest single defeat of the army.  The soldiers were led in a long narrow line through the dense forest that was unfamiliar to them by an officer named Gaius Julius Arminius who was in on the attack, where the troops were systematically attacked and killed while other parts of the line were unaware of any problems.