The 1997 book The Fourth Turning: An American Prophecy proposed a cyclical pattern of four 20-year generations which culminate in a national crisis every 80 years. The book identifies these dates as Fourth Turnings: 1781 (Revolutionary War), 1861 (Civil War) and 1941 (global war). add 80 years and voila, 2021.

I use the term Fourth Turning generically to describe an existential crisis that decisively changes the course of national identity and history.

In other words, we don't have to accept the book's theory of generational dynamics to accept an 80-year cycle. There are other causal dynamics in play that also tend to cycle: the credit (Kondratieff) cycle, for example.

While each of the previous existential crises were resolved positively, positive outcomes are not guaranteed: dissolution and collapse are also potential outcomes.

David Hackett Fischer's book The Great Wave: Price Revolutions and the Rhythm of History proposes another cycle: humans expand their numbers and consumption until they've exploited and depleted all available resources.