-- William Gibson, All Tomorrow's Parties
Pax Americana. Service guarantees citizenship.
Edward Gibbon famously placed the blame on a loss of civic virtue among the Roman citizens. They gradually outsourced their duties to defend the Empire to outside 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. "[T]he decline of Rome was the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the causes of destruction multiplied with the extent of conquest; and as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight," he wrote.
"The Empire had come to depend on the enrollment of barbarians, in large numbers, in the army, and that it was necessary to render the service attractive to them by the prospect of power and wealth. This was, of course, a consequence of the decline in military spirit, and of depopulation, in the old civilised Mediterranean countries. The Germans in high command had been useful, but the dangers involved in the policy had been shown in the cases of Merobaudes and Arbogastes. Yet this policy need not have led to the dismemberment of the Empire, and but for that series of chances its western provinces would not have been converted, as and when they were, into German kingdoms. It may be said that a German penetration of western Europe must ultimately have come about. But even if that were certain, it might have happened in another way, at a later time, more gradually, and with less violence. The point of the present contention is that Rome's loss of her provinces in the fifth century was not an "inevitable effect of any of those features which have been rightly or wrongly described as causes or consequences of her general 'decline.'" The central fact that Rome could not dispense with the help of barbarians for her wars (gentium barbararum auxilio indigemus) may be held to be the cause of her calamities, but it was a weakness which might have continued to be far short of fatal but for the sequence of contingencies pointed out above. (Bury)