Joshua Shanes is associate professor of Jewish studies at the College of Charleston. A popular public speaker, he has published widely on modern Jewish politics, culture and religion.

Today we commemorate Kristallnacht, the “Night of Broken Glass,” a pogrom against Jews throughout Germany and Austria in 1938 that signified a sharpening of Nazi persecution. Mobs destroyed or vandalized hundreds of synagogues, as well as thousands of Jewish homes, schools, businesses and cemeteries. Nearly 100 Jews were murdered, and more than 30,000 Jewish men were put in concentration camps. The move toward the “Final Solution” — the effort to murder every last Jew — would follow just a few years later.

Kristallnacht was not an aberration. It was the culmination of decades of anti-Semitic politics and agitation, mostly expressed and exploited by men who never intended to act on it.

And therein lies the lesson for today.

As anti-Semitic attacks surge in the United States, the media has puzzled over President Trump’s responsibility. Many have concluded that as grandfather to Jewish grandchildren, he could not possibly be anti-Semitic. But what Trump feels in his heart is far less important than what he says and does. And this, too, has historical precedent.

Scholars of anti-Semitism and modern Jewish history are very familiar with the figure of Karl Lueger, the popular mayor of Vienna who rode anti-Semitism to office at the end of the 19th century. Like many divisive politicians today, he almost certainly did not believe in his own rhetoric. Famously, when caught dining with a Jewish friend, he shouted at an anti-Semite, “Wer ein Jude ist, bestimme ich!” (“I decide who is a Jew!”)

But he was a master of what historian Carl E. Schorske called the “politics in a new key,” the politics of rallies, slogans, symbols and emotion. He understood the politics of moving the masses. He abandoned liberalism for populist anti-Semitism because he understood that anti-Semitism could be harnessed rhetorically to electrify and grow his base, to bring in constituents whose economic interests would normally conflict with each other. And he was right.

Modern anti-Semitism, born in the 1870s, acted as a code to rally disparate classes who feared that the effects of modernity — industrialization, urbanization, secularization and globalization — would destroy traditional society and its racial and gender hierarchies. Rather than decrying Jewish religious disbelief, modern anti-Semites feared global domination by an international Jewish conspiracy, blaming Jews collectively for any ill that befell society. Anti-Semites assumed that all Jews constituted a single organism, with a famous Jewish tycoon such as Edmond de Rothschild at its head, conspiring to conquer and destroy the world and its nationalist order. That is why Jews could be seen as both a communist threat and a capitalist exploiter, both the power mogul and his army of poor minions. These were all aspects of changing society.

The problem is that once you stoke and legitimize deep-seated hatred and fear of a minority, it no longer matters whether you believed it or intended to act on it. Anti-Semitism was rendered acceptable in polite (and less-than-polite) society. Lueger and others made it politically acceptable to talk about Jews as an existential threat to the very survival of the nation. And that legacy ultimately helped pave the way for the rise of new anti-Semitic parties in the 1920s and 1930s, groups that did not mean this rhetoric merely as a code but were put into power all the same by voters conditioned to accept such language and soon thereafter the violence that it demanded. When people started acting on those ideas — vigilantes, first, then the government itself — that legitimization played a critical role in the willingness of others to accept it.

Lueger probably would have opposed the violence wrought by his heirs in the 1930s. But they were his heirs all the same. And so is Robert Bowers, suspected of killing 11 Jews praying in their synagogue in Pittsburgh. While his hatred of Jews was partly theological, he mainly repeated textbook myths of modern anti-Semitism. He believed that Jews, collectively as a single being under the guidance of George Soros or other Jewish “elites,” were working to destroy American greatness. And unlike most figures who developed these ideas in the 19th century — but very much like the Nazis in the 20th — he was prepared to act violently to stop them.

Most importantly, he learned those narratives, and felt stoked to act on them, from their constant repetition in a wide array of outlets, from the president on down. He heard the message that good people marched with Nazis. He heard the constant warning of the imminent danger posed by immigrants and the media, by Democrats and anti-fascists and the Jewish oligarchs allegedly funding them, even as he felt frustrated that Trump would not act sufficiently against them.

This does not make anyone else legally liable for this heinous crime. It does, however, highlight the lesson from a century ago: When fearmongering and hatemongering rhetoric designed for political gain is rewarded rather than shunned, it is legitimized. Sooner or later, it will lead to violence, whether by vigilantes who feel empowered or, ultimately, by the state itself. And while Jews are not the primary target of such rhetoric in America today, they are still viewed by many as the collective puppet masters coordinating America’s defeat. When any minority group is under attack, Jews will inevitably be attacked along with them.

Words matter. Hatemongering, fearmongering, violent rhetoric and the dehumanization of people: These are the signs. They must be fought at their first appearance, regardless of the intentions of the politicians who wield them.