My university teacher in ancient history brought us a sheet of paper which read:"Reasons for the decline and fall of the roman empire".
On that sheet of paper where about 70 to 100 terms. Economic crisis, barbarian invasion, incompetence, even weird terms like homosexuallity, but many more then I can recall right now. What she said is, that this are all terms used by historians to explain the decline of Rome. She showed us that for a simple reason, to prove that there is no exact explenation for the decline of Rome.

You people may interpret all the signs and effect, but the decline was so complexe and is far beyond grasping with the limited sources, that we will never fully understand why it happen.

Oh, and the famous historian Habsbawn made an interessting theisis about the decline. He mentions that the cours of history was often determined by civilization against barbarians. He mentions all the ancient empires that were effected by that, but only the Western Roman Empire was completley destroyed. This set out a motion throughout the history of Europe to reinvent and reconstruct the Roman Empire, most notably the Resnaissance. Thus Europe set out for a different path of history. It's a bit vague how I can discribe it, but if someone is interessted in the passage I can type it down.