Polybius and anakyklosis, the cycle of constitutions
Every simple regime decays into its own worst version, in a fixed order. Rome held the cycle off for a while by mixing all three at once.
Polybius, Histories VI 1377 · Tunis and CairoIbn Khaldun on why dynasties die in four generations
Political power rests on solidarity forged by hardship. Cities dissolve it, comfort finishes the job, and a dynasty lasts about four generations.
Ibn Khaldun, Muqaddimah c. 1517 · FlorenceMachiavelli and the ten-year cure for corruzione
Rome stayed free because its classes fought openly. Machiavelli read conflict as maintenance rather than disease, which nobody else was willing to say.
Machiavelli, Discourses on Livy 1918 · MunichOswald Spengler on why political orders decay
The most famous book of decline of the last century, and the one to handle with tongs. What the morphology claimed, why specialists rejected it, and who picked it up afterwards.
Spengler, The Decline of the West 1930 · MadridOrtega y Gasset and the hombre-masa
The mass-man is a temperament rather than a class: the person who enjoys a civilisation without asking what maintains it, and who appears in the professions as often as anywhere.
Ortega y Gasset, The Revolt of the Masses 1988 · ArchaeologyJoseph Tainter and the cost of complexity
Complexity is a problem-solving strategy with diminishing returns. When they go negative, collapse stops being a catastrophe and becomes the rational option.
Tainter, The Collapse of Complex SocietiesStart here instead
If you want the whole tradition in one place rather than one thinker at a time, the long argument about why civilizations fall runs from Polybius through Peter Turchin and asks which side holds up.