Ah, yes, Parallel Pain. "Tribes fighting tribes" indeed. We tend to look at Rome as "Romans," but overlook the fact that in the dawn of their history (the legend of Romulus, the wolf-stepmother, etc.), they were different tribes, each on top of their own hill of the Seven, squabbling over grazing rights, water access, what have you. And they do indeed have their animosities towards each other. Then, later, it was Latins vs. Sabines, Etruscans, (Italic) Greeks, and so forth. Then the Gauls, then . . . Warfare is a tribal thing. When they were on the road to expanding their empire (to give us a hint as to why the Empire began to fall), one excuse the Romans had was to make a preemptive war on her neighbor--to have a buffer territory between Roman land and their imagined enemy "just over the next hill." Then take the next neighboring land as the next "buffer territory" and so on. With such expansionism, of course the Empire would have many peoples, many tribal sentiments, . . . many dieties . . . As Gaius Curio and macsen rufus say above, political infighting, internal pressures simultaneous with external threats from the barbarians (remember I said Rome was both a delicious prize both for its riches as well as adding glory to the personal pride of the conquering warlord?) were contributory. The thing about lead poisoning that I suggested was just one -a minor one at that- of the hundreds of factors that lead to the demise of the Empire. Maybe I could also suggest Christianity as one of the contributors: as a Christian you are taught to love your enemies; to trust in heavenly powers (a popular legend says Constantine saw a vision of the cross, the reason why he was victorious in that battle at that bridge) instead of the strength of your sword-arm to defeat one's foes, etc. It takes plenty of forces and stresses to kill an empire--compare this with the "fading" of the British Empire, or the Spanish Empire, or even the Assyrian, Babylonian, Chaldean, Persian, Egyptian . . . empires. Spot the similarities of why they fell (or faded). What we touch in this forum--this thread--are only a few of those factors. But we do have a fun discussion here, and it DOES fuel us to do research and interact with one another, and that's good.
Bye-bye, noble friend--till the next time, take good care of yourself and yours.