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.