Final answer:
The myth of Roman invincibility was shattered at the Battle of the Teutoburg Forest in 9 CE and further eroded when the Goths breached Rome's walls in 410 CE, for the first time in seven centuries.
Step-by-step explanation:
The myth of Roman invincibility was destroyed at the Battle of the Teutoburg Forest in 9 CE. This significant event saw the annihilation of several Roman legions and stopped Roman expansion across the Rhine. The detailed accounts of this battle, occurring over three days, provide a rare insight into the complete collapse of a Roman army.
Moreover, the Gothic attack on Rome in 410 CE, breaching the city's walls for the first time in 700 years since the Celtic sack in 387 BCE, significantly undermined the narrative of Roman impregnability, causing shock and horror throughout the Roman world.