“[Columbia professor Adam Tooze, writer of the definitive forensic analysis of the 2008 financial crisis in Crashed: How a Decade of Financial Crises Changed the World,] does not buy the line that America is roaring back at the head of a resurgent West, even if the autocracies have suffered a crushing reverse over recent months. ‘I see America as the huge weak link,’ he said.
He broadly subscribes to the Fukuyama thesis that the American body politic is by now so rotten within, so riddled with the cancer of identity politics that it is developing a paranoid loser’s view of the world. The storming of Congress was not so much an aberration under this schema, but rather the character of modern America.”
–Ambrose Evans-Pritchard, “The world’s financial system is entering dangerous waters again, warns guru of the Lehman crisis.” The Telegraph. May 23, 2022.
Open question: Is the current populism and “paranoid style” of the American character an sign of decline or a trait that becomes more prevalent with populist resurgence?
The paranoid character of U.S. politics is not a new claim, see the Richard J. Hofstadter essay, “The Paranoid Style in American Politics.” The online version is Harper’s Magazine is currently behind a paywall. But, I’d imagine most city public libraries have a copy of it.
The paranoid style is a recurring feature of populist movements, right and left, evident from so-called militia/patriot movements to the “woke” left of our time. Nothing is really new about either. But, is there something new in this wave? Is it significantly different than movements that led to prohibition of alcohol and marijuana?
I’m inclined to see the current environment as a variation on a consistent pattern, like the Great Awakenings. Ultimately, these kinds of heated discussions are the strength of democracies, even when they lead to things like the U.S. Civil War. You get your say. If you feel strongly enough, you fight about it. But, in the end, a decision is made and you see how it goes. It’s not dictated by some clown at the top. It’s messy. But, it’s better than the alternative.
cafebedouin.org 
I initially misread your title as “America’s Modal Character,” which seems equally accurate.
I’ve always believed that, for the Western world (especially Europe), the greatest inflection point that broke its character (as though it was unified) was The Great War (WWI), the first mechanized war. It was a change of sea from which we have arguably never recovered. More locally (time and place), the most severe inflection point for the U.S. was 9/11, which single-handedly sent the U.S. into a psychological tailspin where identifying as victims (losers) became commonplace. Nearly every geopolitical move since has been from that perceived position of weakness, which is of course bogus but allows us to rationalize our crimes against the rest of the world.
I might argue that the Civil War was the first industrialized war. Given how prominently it features in some people’s contemporary rhetoric, it might be a useful reference point for the problems in the American character. You could trace back Jim Crow, reactions against labor, social movements like Progressivism, the McCarthy Red Scare is moving out from an internecine war to one international in scope, and 9/11 was a kind of cherry on top for that type of thinking. But, your comment is a good way of thinking about it too.