It is remarkable, in retrospect, how many and varied were the dictatorships of the past century.  Murderous regimes . . . [from] wealthy Germany and impoverished Rwanda [to] Buddhist Cambodia and Orthodox Russia.

Yet these different regimes did all have one thing in common . . . the “mania for classification and elimination of different elements of society.” In each one of them, the groundwork for violence against a specific group — whether an ethnicity, an economic class or a political faction — was originally laid by a very particular way of using language.

In the first instance, inflammatory language was used to define an ethnic minority and to give it fictional characteristics and properties. In some cases, the targeted “tribe” was entirely fictional, created by rhetoric alone. . . .

After the unwanted group had been defined, propaganda was used to demonize and dehumanize it. In the Soviet Union, Vladimir Lenin characterized Russia’s ex-rulers as “former people,” as if their humanity had somehow been dissolved by the revolution. Germany’s Adolf Hitler and Russia’s Joseph Stalin went further, describing unwanted categories of human beings as “vermin” or “parasites” or “poisonous weeds.” The Nazis even made posters, depicting Jews as lice.

For the past half-century, memory of where it once led has made this kind of language taboo in Western democracies. Now it is undeniably back. I am not comparing President Trump or his European counterparts to Lenin or Hitler; even to do so gives all of them a significance they don’t deserve. But they have brought back the “mania for classification and elimination of different elements of society,” and this will have real consequences.

It is worth noting how often the president repeatedly conflates refugees with illegal immigrants and MS-13 gang members.

[It is also worth noting, I would think, that Trump kept a book of Hitler’s speeches by his bedside. — A.T.]

This is not an accident: He has targeted a group and given them characteristics — they are violent, they are rapists, they are gang members — that don’t belong to most of them. He then describes them with dehumanizing language. Democrats, he has tweeted, “want illegal immigrants, no matter how bad they may be, to pour into and infest our country, like MS-13.” The image of “infestation” evokes, again, vermin and lice. A few weeks earlier, he spoke of MS-13 as “animals,” once again making it unclear whether he meant actual gang members or simply those who distantly resemble them.

Trump isn’t alone. Matteo Salvini, the new Italian interior minister, also recently spoke of “mass cleansing” of “entire parts” of Italy. He was speaking of Roma — gypsies — but again in a way that was unclear. He conflated “Roma” with “foreigners” and then with “children who have been taught to steal.” A bit farther east, the Hungarian prime minister Viktor Orban has achieved an even greater feat: He has whipped up fear of a target group — foreign immigrants — who do not actually exist in Hungary.

I don’t believe any of these leaders are, at the moment, planning mass murder. The purpose this time is different: to define and classify a group whose existence can be used to create fear. Social media can be used to give these enemies greater numbers than they have in reality; even when they don’t exist, talk of “no-go zones” and “crime waves” can be used to win votes.

These tactics will produce casualties. The border police who took children from their parents at the U.S.-Mexico border were mentally prepared to do so thanks to the language of dehumanization. . . .  They will also produce imitators and amplifiers, such as the Oregon woman who called for immigrants to be shot at the border, or the Fox News pundit who said there was no need to worry about those children because, after all, they aren’t American. . . .