“Democrats are the problem,” Donald Trump tweeted on Tuesday. “They don’t care about crime and want illegal immigrants, no matter how bad they may be, to pour into and infest our Country, like MS-13. They can’t win on their terrible policies, so they view them as potential voters!” Like many of Trump’s tweets, this statement contains both a lie and a slur. The Democratic Party bears some responsibility for the Obama administration’s immigration policies—humane and restrictionist alike—but it did not create the zero-tolerance policy that now breaks up families at the border. That policy belongs to Trump; he will be remembered for it, and for the rhetoric he has used to justify it.

“Infest” is not a word customarily used in reference to humans. We fear infestations of cockroaches or bedbugs or lice. To accuse people of infesting America is to dehumanize them.

This is not the first time Trump has referred to migrants in this way. Sanctuary cities, Trump once complained, are “crime-infested,” as if the presence of immigrants guarantees an outbreak of violence. He is also worried about their “breeding,” a word usually reserved for animal or insect mating: “Soooo many Sanctuary areas want OUT of this ridiculous, crime infested & breeding concept,” he tweeted in April.

Trump did not invent this language from whole cloth. Modern history is full of examples of political regimes that has described certain populations as subhuman—often to justify treating them as such. In the most extreme cases, that rhetoric preceded mass killing.

“One of the common threads of any genocide is its justification. In order to be able to execute it on a mass scale, a lot of people have to buy into it and agree that it’s the appropriate thing to do. And so any genocide begins with the dehumanization process,” explained William Donohue, a professor of communication at Michigan State University who has studied the function of dehumanizing language in the Rwandan genocide.