Most Of Us know,

that eventually the Roman Empire was overrun and Rome sacked, 

detail of The Sacking of Rome, Karl Bryullov, 1833-1836, in the State Tretyakov Gallery, Russia, the city has been in existence for over 2,000 years, and following the Roman Empires expansion picked up a few enemies on the way, leading to its eventual downfall, but the city was not sacked just once, as it happens it was overrun eight times, the city was taken by the Galls, the Visigoths, the Vandals, Germanic barbarians, the Ostrogoths, Arab raiders, the Normans, and the Holy Roman Empire, if the last one seems like a civil war, it sort of was but it included most of Europe marching against Rome, it appears that to be a successful invader it pays not to upset the locals you invade! for the full rundown on these events and indeed as to how Rome rose again after them have a look here.