As overwhelmingly efficient and successful as it was, the Roman Army was not beyond getting a bloody nose now and then. Hannibal’s victory at the Battle of Cannae (216 BCE) almost led to the collapse of Rome, and Crassus’ loss to the Parthians at the Battle of Carrhae (53 BCE) made possible the civil war between Caesar and Pompey that eventually led to a change from republic to empire. However, it was the crushing of three legions under Varus in 9 CE, that shook the fledgling empire under Augustus, to its core.
Relationships between the northern region of the Roman Empire, and the tribes occupying Magna Germania north of the Rhine, were always complicated, shifting, and prone to violent outbreaks. The Roman governor of the region, Publius Quinctilius Varus, seemed to have a knack of annoying the locals wherever he was stationed, however, his downfall came from within.
Arminius was a prince of the Roman-friendly part of the Cherusci tribe, and fought in the Roman Army, learning Latin, and reaching the high societal rank of Eques. However, on being sent to Varus to aid in the subjugation of the hostile tribes, he plotted a revolt against Rome, which bore fruition in the ambush of three legions in the Teutoburg Forest, annihilating them. The Roman general Germanicus was sent to recover the situation, which he largely accomplished, but the Teutoburg defeat effectively prevented further Roman expansion in the region. As for Arminius, other tribal leaders were wary of his growing power, and assassinated him.