Toynbee argues that the collapse of civilisations is ultimately driven by two closely related forces: destructive warfare and conflict between social classes. Looking across the civilisations recognised in his historical scheme, he claims that their disintegration can consistently be traced to one of these causes or to a combination of both.

The argument places internal human conflict, rather than external fate or material scarcity alone, at the centre of civilisational decline. War consumes resources and weakens social institutions, while class conflict undermines cohesion within a society. The two can also reinforce one another, as inequality and internal division contribute to instability while prolonged warfare intensifies social pressures. Toynbee presents this as a broad historical principle and implicitly as a warning for Western civilisation: continued survival cannot be assumed if it remains vulnerable to warfare and deep social division.