I'm reading Edward Gibbon's book on the decline and fall of the Roman Empire and he seems surprised the Roman Empire lasted as long as it did. He also states the introduction to Christianity and its abuse seems to have been the beginning of the end of the Empire.
The FAQ section on the decline of the Roman Empire might be of interest to you, as might the section on Gibbon, whose work, while a literary masterpiece, is considered wildly out-of-date (to put it mildly).