The startling aspect about the Dolchstosslegende is this: It did not grow weaker after 1918 but stronger. In the face of humiliation and unable or unwilling to cope with the truth, many Germans embarked on a disastrous self-delusion: The nation had been betrayed, but its honor and greatness could never be lost. And those without a sense of national duty and righteousness — the left and even the elected government of the new republic — could never be legitimate custodians of the country.

In this way, the myth was not just the sharp wedge that drove the Weimar Republic apart. It was also at the heart of Nazi propaganda, and instrumental in justifying violence against opponents. The key to Hitler’s success was that, by 1933, a considerable part of the German electorate had put the ideas embodied in the myth — honor, greatness, national pride — above democracy.

The Germans were so worn down by the lost war, unemployment and international humiliation that they fell prey to the promises of a “Führer” who cracked down hard on anyone perceived as “traitors,” leftists and Jews above all. The stab-in-the-back myth was central to it all. When Hitler became chancellor on Jan. 30, 1933, the Nazi newspaper Völkischer Beobachter wrote that “irrepressible pride goes through the millions” who fought so long to “undo the shame of 9 November 1918.”

Germany’s first democracy fell. Without a basic consensus built on a shared reality, society split into groups of ardent, uncompromising partisans. And in an atmosphere of mistrust and paranoia, the notion that dissenters were threats to the nation steadily took hold.

Alarmingly, that seems to be exactly what is happening in the United States today. According to the Pew Research Center, 89 percent of Trump supporters believe that a Joe Biden presidency would do “lasting harm to the U.S.,” while 90 percent of Biden supporters think the reverse. And while the question of which news media to trust has long split America, now even the largely unmoderated Twitter is regarded as partisan. Since the election, millions of Trump supporters have installed the alternative social media app Parler. Filter bubbles are turning into filter networks.

In such a landscape of social fragmentation, Mr. Trump’s baseless accusations about electoral fraud could do serious harm. A staggering 88 percent of Trump voters believe that the election result is illegitimate, according to a YouGov poll. A myth of betrayal and injustice is well underway.

It took another war and decades of reappraisal for the Dolchstosslegende to be exposed as a disastrous, fatal fallacy. If it has any worth today, it is in the lessons it can teach other nations. First among them: Beware the beginnings.

Jochen Bittner (@JochenBittner) is a co-head of the debate section for the weekly newspaper Die Zeit and a contributing opinion writer.

The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. And here’s our email: letters@nytimes.com.

Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram.