Arnold Toynbee

From Wisdom Supreme:

In his Study of History Toynbee describes the rise and decline of 23 civilisations. His over-arching analysis was the place of moral and religious challenge, and response to such challenge, as the reason for the robustness or decline of a civilisation. He described parallel life cycles of growth, dissolution, a "time of troubles," a universal state, and a final collapse leading to a new genesis. Although he found the uniformity of the patterns, particularly of disintegration, sufficiently regular to reduce to graphs, and even though he formulated definite laws of development such as "challenge and response," Toynbee insisted that the cyclical pattern could, and should, be broken.

Lots of Toynbee quotes at the above link. And the first four short paragraphs, under "Ideas", by the old man's picture, give a good overview of his thinking.

And from the Catholic Education Resource Center:

The historian Arnold Toynbee observed: "Out of 21 notable civilizations, 19 perished not by conquest from without but by moral decay from within."

Not sure about the 23/21 discrepancy. Either way it's a lot of civilizations, all following a pattern Toynbee lays out in great detail.

Obviously I think he's wrong about "moral decay" being the source of decline. People often see economics and say morality.