Arminius , also known as Armin or Hermann ( b. 18 BC\/17 BC in Magna Germania ; d. AD 21 in Germania ) was a chieftain of the Cherusci . Arminius , also known as Armin or Hermann ( b. 18 BC\/17 BC in Magna Germania ; d. AD 21 in Germania ) was a chieftain of the Cherusci who defeated a Roman army in the Battle of the Teutoburg Forest .