Why Did Rome Fall?
Or, to be more precise, why did the Western half of the Roman Empire fall? I'm well aware that the Eastern portion remained in existence until 1453, but the fact is that the civilization of which we are today a part, and whose influence predominates throughout the world, was born of the ashes of the Western half that collapsed in 476, rather than of the Byzantine state: as such, the continued existence of that other civilization, though undoubtedly of interest to historians, is, as a practical matter, of far less importance to understanding how our world came to be the way it is today.
Although I'm familiar with the sorts of moral reasons adduced by Edward Gibbon for the fall of Rome - enervating oriental luxury, sexual immorality, etc. - I'm not at all convinced that "moral decline -> civilizational decline" is in any way relevant to understanding what really went on. From my own reading, the correct answer must lie in some way with the economic conditions of the empire - in particular, with the ubiquitous institution of slavery, and the effect that the cessation of Roman expansion, and therefore, opportunities to seize vast new numbers of slaves, must have had on the economic viability of slave-based industry and agriculture. Ultimately, I think the attitude learned from the Greeks that work was something to be despised is the root cause of the decline that set in well before the barbarian hordes finally knocked down the rotting edifice.
If I am correct, there's a lesson in there for those on the left who take the attitude that there's something dishonorable about striving "too hard" to better one's material lot, and that leisure is more "civilized" than, and always to be preferred to labor, an attitude that is now commonplace throughout Europe, particularly amongst the more comfortable classes.