Economic History: Is it in fact the case that once an agrarian civilization starts to lose resources, it is headed for collapse or near-collapse? It is certainly true that something must make the élite accept life on a reduced total resource base—either conquest and wholesale élite turnover, major winnowing via civil war, purges, or gatekeeping accession to élite status—and that all such methods impose further burdens. But, at least as I read it, “fall” proper is confined to the end of the Bronze Age, the Maya, and to the Roman provinces of Britain, Germany, Gaul, and (perhaps) Spain. Decline is common. Fall not:

Peter Turchin & Sergey A. Nefedov: Secular Cycles <press.princeton.edu/boo…>: ‘A gentlemen living on £10–20 a year who was employing only three servants and lived in one house, and whose meals were devoid of much luxury in terms of wine and spices, had little room to maneuver when his income plunged by up to 50 percent in the mid-fifteenth century: “They must have cut back, or even cut out completely, their occasional wine-bibbings, and avoided travel whenever possible, but too many economies of this kind might force them to drop out of the aristocracy and accept yeoman status” (Dyer 1989:108). In summary, a number of social mechanisms exist by which  elite surpluses can be reduced: (1) deaths resulting from civil war, (2) deliberate purges of elites by new rulers, (3) limitations imposed on heir production... [Turchin, Peter, and Sergey A. Nefedov. 2009. Secular Cycles. Princeton: Princeton University Press. <press.princeton.edu/boo…>.]