Well, there were many signs of the coming decline. Rather than relist a ton of history and do it very sketchily, which will no doubt irritate a number of pedantic types here, why not just list a series of specific problems that arose in very brief form?

- increasing political corruption at every level

- increasing power and wealth in the hands of an elite few

- expansion of the empire in all directions to the point where it simply became too large and unwieldy to manage itself efficiently

- corruption of popular entertainment. A public once satisfied by watching plays and chariot races became a public that wanted to be entertained by watching horribly bloody gladiatorial combats, people being torn apart by wild animals, etc.

- increasing corruption of private life among the wealthy classes, including some really bizarre sexual practices, eating and drinking so much as to induce vomiting, then eating more, etc...

- bottling wine in lead-lined bottles which eventually caused lead poisoning and progressive insanity to occur in a good many people, specially the wealthiest people, I would think.

- military committments that were so enormous that it began to eat up the wealth of that society in ways which were unrecoverable

- dilution of the value of Roman currency

- depletion of local resources, which required further conquering of outlying resource-rich areas, which required further spending on military forces, which required maintaining longer lines of communication and making more enemies....

Basically what killed Rome was what has killed virtually all huge, aggresive imperial empires who became number 1 in the world of their day. They simply took it too far. They conquered too much. They invaded too many places. They became fiscally irresponsible. They became morally irresponsible. Their government became autocratic and utterly corrupt at the highest levels. They lost touch with the aspirations of the common people and provided no real moral leadership. They became self-indulgent and self-absorbed to a degree where they lost touch with reality. And then they became undisciplined, unrealistic, and weak. They became a sick society that no longer had any real ideals, where nothing counted anymore but "winning"...and winning simply meant: being in power and being very, very rich.

Such empires always fall. The only question is when. And how. And how fast.