A grand coalition of barbarian tribes attacked the Roman empire at its height and brought Caesar to his knees, the little bitch. Even though Rome was united and had a very strong sense of Roman identity at the time, and wasn't just a bunch of barbarian tribes slapping the label "Rome" on because some merchants in the cities told them to. It could not stand against the might of the Barbarian Coalition (as it is called in historical circles today). This is how Rome fell.