[faidit]

The world war helped via wealth destruction and redistribution. Polybius' theory also has Arabic (Khaldun) and Chinese analogues. In China it looks like a 300-ish year cycle driven by wealth/land concentration and elite corruption which ends with peasant revolts, a power vacuum, peasants taking back their own land, and a new dynasty formed by rebels or invaders, after which the new elite slowly begins encroaching on the peasants and taking their land again.

[constantius]

The idea that wars, epidemics, and catastrophes lead to wealth redistribution is the thesis of "The Great Leveler" by Walter Schneidel: it's not an absolute rule (and as far as I remember, another book was published to criticise this one), but it seems to hold through a change in power dynamics, supply and demand of labour, and coordination between the previously oppressed classes.