Edward Gibbon famously placed the blame on a loss of civic virtue among the Roman citizens. They gradually entrusted the role of defending the Empire to barbarian mercenaries who eventually turned on them. Gibbon considered that Christianity had contributed to this, making the populace less interested in the worldly here-and-now and more willing to wait for the rewards of heaven. http://en.wikipedia.org/wiki/Fall_of_the_Roman_Empire Впрочем, более убедительной выглядит версия (из множества), что в 165-180, и затем в 251-266 половину населения скосила чума, а последующие военные, политические и экономические факторы - результат.
