What were the major reasons for the fall of the Roman Empire?

Historians have argued about the reasons for the fall of the empire, but there is no one single reason. A few possible reasons are that the empire was overextended; its political organization was not sufficient to manage an empire of that size; there was significant difficulty incorporating the diverse peoples of the empire and maintaining loyalty in the dominate period; there were several economic crises in conjunction with the political crises of the third century; and there was political fragmentation in the sense of localization taking priority over the centralized, urban lifestyle of the empire at its prime. Some historians argue that Augustus's demobilization of military forces in the first century had depleted the army of the strength necessary to fend off barbarian tribes; others argue that Christianity had a weakening effect on the overall unification mentality of the Roman Empire.