Civilizations, rise, flourish and fall. We all know that.

Apart from the 'Hard times create strong men, etc' axiom, there is currently a more sofisticated approach based on IQ as layed out by Michael Woodley of Menie and Edward Dutton (https://www.amazon.co.uk/At-Our-Wits-End-Intelligent/dp/184540985X/ref=sr_1_1?keywords=dutton+woodley+of+menie&qid=1580158568&sr=8-1)

Since IQ is the best predictor for success for an individual, a society's IQ is the best predictor for the success of a society, or civilization, as explained in the above linked book.

Example 1: the bubonic plague killed approximately one third of the population in Europe, estimates are up to half the population were killed in the Mediterranean. Who dies? The poor, the sick, the stupid. Consequently the general IQ of those that survive is higher. About 200 years later, you get the Renaissance.

Example 2: in Britain all had many children, but less children of the poor survived compared with the children of the rich and successful. Consequently, the IQ rises in this survival of the richest. A couple of centuries later, you get the Industrial Revolution with an unprecedented amount of inventions. As a result, living standards rise and more poor children survive, dropping the society's IQ again. We are now back at the same level as in the 1600's they state.

On top of that, the increasing amount of surviving mutant genes negatively influence the remaining 'good' genes.

The explanation is very logical, very Darwinian, tough as a nail and, I think, very convicing. If you haven't heard of it yet, I think it will interest you. They have an entertaining YouTube channel also: The Jolly Heretic.