In case after case we will see that when societies fail to solve the problems that confront them, a terrible package of ills—famine, epidemic, uncontrolled migration, and state failure—begins to afflict them, turning stagnation into decline; and when famine, epidemic, migration, and state failure are joined by further forces of disruption, like climatic change (collectively, I call these the five horsemen of the apocalypse), decline can turn into disastrous, centuries-long collapses and dark ages.
