From the link you just posted: After serving with distinction in the Great Illyrian Revolt, he was sent to Germania to aid the local governor Publius Quinctili

From the link you just posted: After serving with distinction in the Great Illyrian Revolt, he was sent to Germania to aid the local governor Publius Quinctilius Varus in completing the Roman conquest of the Germanic tribes. While in this capacity, Arminius secretly plotted a Germanic revolt against Roman rule, which culminated in the ambush and destruction of three Roman legions in the Teutoburg Forest.