There wasn't a single thing that brought Rome to its knees, but instead a dastardly combination of several factors. At the source of everything, the Roman government was incredibly corrupt and ineffective, and Roman citizens did not trust in the leaders of the Republic. In addition, the rise of Christianity and the shying away from the Roman polytheistic religious traditions also contributed, as the latter viewed the emperor as having deistic authority. Finally, because of the sheer size of the Empire, it was incredibly hard to govern on the fringes and the Roman Empire spent too much money bolstering its military might.
