Book 5, Chapter 1, beginning at the second paragraph:
It may be observed, that provinces amid the vicissitudes to which they are subject, pass from order into confusion, and afterward recur to a state of order again; for the nature of mundane affairs not allowing them to continue in an even course, when they have arrived at their greatest perfection, they soon begin to decline. In the same manner, having been reduced by disorder, and sunk to their utmost state of depression, unable to descend lower, they, of necessity, reascend; and thus from good they gradually decline to evil, and from evil again return to good. The reason is, that valor produces peace; peace, repose; repose, disorder; disorder, ruin; so from disorder order springs; from order virtue, and from this, glory and good fortune. Hence, wise men have observed, that the age of literary excellence is subsequent to that of distinction in arms; and that in cities and provinces, great warriors are produced before philosophers. Arms having secured victory, and victory peace, the buoyant vigor of the martial mind cannot be enfeebled by a more excusable indulgence than that of letters; nor can indolence, with any greater or more dangerous deceit, enter a well regulated community. Cato was aware of this when the philosophers, Diogenes and Carneades, were sent ambassadors to the senate by the Athenians; for perceiving with what earnest admiration the Roman youth began to follow them, and knowing the evils that might result to his country from this specious idleness, he enacted that no philosopher should be allowed to enter Rome. Provinces by this means sink to ruin, from which, men’s sufferings having made them wiser, they again recur to order, if they be not overwhelmed by some extraordinary force. These causes made Italy, first under the ancient Tuscans, and afterward under the Romans, by turns happy and unhappy; and although nothing has subsequently arisen from the ruins of Rome at all corresponding to her ancient greatness (which under a well-organized monarchy might have been gloriously effected), still there was so much bravery and intelligence in some of the new cities and governments that afterward sprang up, that although none ever acquired dominion over the rest, they were, nevertheless, so balanced and regulated among themselves, as to enable them to live in freedom, and defend their country from the barbarians.
Political theorists must recognize it for it is a foundational truth; one not much different than the law of gravity.
From good civilizations come weak men; from weak men come bad civilizations, from bad civilizations come strong men; from strong men come good civilizations.
The golden question is how do we stop it?