Peter Turchin

“The collapse of order brings in its wake the four horsemen of the apocalypse - famine, war, pestilence, and death. Population declines, and wages increase, while rents decrease. As incomes of commoners recover, the fortunes of the upper classes hit the bottom. Economic distress of the elites and lack of effective government feed the continuing internecine wars. But civil wars thin the ranks of the elites. Some die in factional fighting, others succumb to feuds with neighbors, and many just give up on trying to maintain their noble status and quietly slip into the ranks of the commoners. Intra-elite competition subsides, allowing order to be restored. Stability and internal peace bring prosperity, and another secular cycle begins. 'So peace brings warre and warre brings peace.”
― Peter Turchin, War and Peace and War: The Rise and Fall of Empires

“We also know, of course, that human beings are not perfectly rational calculators. Our behavior and decisions are based on a mixture of calculation, emotions, and internalized norms, with calculation often a minor component of the cocktail.”
― Peter Turchin, Ultrasociety: How 10,000 Years of War Made Humans the Greatest Cooperators on Earth