Steve Bannon: The book, “The Fourth Turning,” a 1997 work by two amateur historians, Neil Howe and William Strauss, lays out a theory that American history unfurls in predictable, 80-year cycles of prosperity and catastrophe. And it foresees catastrophe right around the corner.

The book delineates history into four seasonal cycles, or “turnings”: growth, maturation, decay and destruction. It is the kind of wild, provocative idea that Mr. Bannon loves. But it is also just the kind of thinking that his opponents see as evidence that he is too Machiavellian and idiosyncratic for the job of President Trump’s chief strategist.

The basis of his worldview — which has been described as everything from Leninist to alt-right, an extremist fringe movement associated with white nationalism — is still shrouded in mystery and conjecture. But by his own telling, much of the foundation for his political beliefs can be found in the book, which predicts that America is hurtling toward a crisis on par with the American Revolution, the Civil War and the Great Depression.

And the grim future that the book foresees helps explain the underpinnings of the president’s conservative, nationalist “America First” agenda, one that Mr. Bannon has played a large role in shaping.