Sociopolitical collapse is the rapid simplification of a society. In historical cases, cities and monuments quickly disappeared, population diminished, and there was pronounced loss of sociopolitical complexity. Famous examples include the collapses of Mycenaean Greece in the 12th century B.C., the Western Roman Empire in the 5th century A.D., and the Classic Maya in the 9th century A.D. The periods that follow are sometimes called Dark Ages. Every society is characterized by a degree of complexity, which describes how extensively it is differentiated by roles, specialization, technology, and information, and what institutions regulate life. More complex societies are most costly to institute and maintain than simpler ones, requiring high levels of energy. Simplified post-collapse societies needed and produced less energy per capita. Before fossil fuels, societies were powered largely by solar energy. In such societies, greater complexity meant that people worked harder. Photosynthesis produces little energy per unit of land (maximum of about 170 kcal/sq m/year). The weakness of ancient complex societies was that peasants, whose taxes funded government and the military, lived on very small margins of net agricultural production. Ancient societies often faced problems, especially war, that required greater complexity and energy production. Taxes compelled peasants to produce more, but excessive taxation could undermine peasants well-being and ultimately the society itself. Vulnerability to collapse often emerged from...