476 is the significant date, most accept.. When the last Roman Emperor to rule from Rome, was deposed by a barbarian Chieftain.. The "Empire", of the Caesars, didn't rise again..
During the age of the Republic/Empire, I believe Rome was sacked 3 times, Gauls, Visigoths, and Vandals.. But it happened after the the end of the Empire as well..
That is what I refer to, but I guess it could be open to historical interpretation..
Point is, after a long glorious period, Rome lost it's grip on power.. Was rejuvenated for a
short period of time, and then faded away!