Malthus and the Disintegration of Empires.
Peter Turchin styles himself as a latter day Hari Selden – he is looking for theories of Cliodynamics - general principles of history. Two big principles animate his War and Peace and War. One is due to the great Arab historian and polymath Ibn Khaldun. Khaldun identified the crucial role of asabiya – the fundamental social glue that unites a people – in the stability of nations and empires. The second is the role of Malthusian cycles in the instability of empires.
The basic idea of the Malthusian cycle is that peace and prosperity lead to growth in the numbers of the peasant class. This leads to competition for land, increases in rents, decreases in pay for landless laborers and increased prosperity for the nobles and other rentiers, which, in turn, leads to an expansion of the Noble class. The peasants and laborers suffer starvation, plague, and the other apocalyptic catastrophes leading to collapse of their numbers. Now it is the turn of the newly bloated upper class to take their turn in the Malthusian meat grinder. Rents collapse so less and less is there to be shared by more and more. Internal strife and external war plunge the nation into internal discord, destroying the social capital of asabiya.
If this sounds like classic predator-pray dynamics, it is. Turchin studied beetles before he turned to humans, but he has the numbers, for England, France, Rome, and other imperial nations.
One more ingredient comes from computer models of trade and inheritance: trade and inheritance produce inequality, the so-called Mathew Principle. The rich get richer, and the poor get poorer, statistically speaking. In the most potent days of the Roman Republic, the rich Romans had about 5 times the wealth of the median. By the collapse, that ratio had grown to 200,000. For comparison, today Jeff Bezos has something like three million times the wealth of the median American.
Is it any wonder that American asabiya is collapsing?