A Class War Marks The Start Of A Civilization's Decline
from 'A Study Of History' by Arnold Toynbee (1952)

Part V: The Disintegration Of Civilizations — Chapter 27 The nature and symptoms of social disintegration

On the other hand, the horizontal schism of a society along lines of class is not only peculiar to civilizations, but it is also a phenomenon which first appears at the moment of breakdown, and which is a distinctive mark of the phases of breakdown and disintegration, by contrast with its absence during the phase of growth.