With his promise to revoke, by executive order, the guarantee of birthright citizenship in America, President Trump has made the inconceivable possible. Most legal scholars appear to agree that the Fourteenth Amendment cannot be changed in the way that Trump proposes, but his vow has already elevated previously marginal arguments—they are now positions to be considered. If the President follows through with an executive order, what may have seemed like legal nonsense yesterday will have to be weighed by the courts. We have seen this mainstreaming of previously unimaginable ideas many times in the last two years: a ban on Muslims entering the country, the border wall, the withdrawal from the Paris climate accords, and more.

The idea of revoking birthright citizenship is consistent with Trump’s newly vigorous embrace of the label “nationalist.” Most recently, at a rally in Houston and in an interview with Fox News, Trump has defined “nationalist” as the opposite of “globalist.” The Times columnist David Brooks criticized Trump’s use of the word by arguing that he loves America more than the President does; both men use the word “nationalist” as though it were synonymous with “patriot,” but this shift in usage is significant. “Nationalist” suggests a country under siege and carries the connotations of thinking of the United States as a nation-state that is ethnically, culturally, and religiously homogeneous. This, in turn, is consistent with the removal, in February, of the words “nation of immigrants” from the mission statement of United States Citizenship and Immigration Services. As with so many things, we haven’t talked nearly enough about how much these shifts in official rhetoric change the dominant political story.

When Trump said, on “Axios on HBO,” that the U.S. is the only country in the world where people obtain citizenship by virtue of being born, he lied. Birthright citizenship is the rule in the Americas. Many Western European countries allow people who were born there to apply for citizenship once they turn eighteen. Some countries are more restrictive. In Germany, for example, a newborn is considered a citizen only if at least one of the baby’s parents is a legal resident who has been in the country for more than eight years. This law is a source of shame for many Germans, precisely because it is rooted in ideas of a nation’s ethnic and cultural purity. These kinds of laws create an ever-growing class of disenfranchised people who live in Germany legally and can participate in the economy but not in national politics—and this is precisely Trump’s objective, too: to shut Americans whom he perceives as other out of the political system. Two years after claiming, obsessively and falsely, that millions of “illegal immigrants” voted in the Presidential election, Trump is taking steps to make immigrant votes illegal.

The pernicious effects of Trump’s statement do not end there. The only reasonable response to his attack on birthright citizenship is to defend it; the problem is it is indefensible. It may seem like a self-evident right, but it is based on a decidedly premodern premise. As the Northwestern University professor of political science Jacqueline Stevens has argued, in an online debate organized by The Economist, “Just as it would be unacceptable for a government to announce in advance that at birth one’s options to attend Oxford, earn $1m, or run for Mayor of London will be reserved only to those able to claim ancestors with these attributes, anyone claiming to embrace liberal values should find it equally unacceptable to use birth—either in a geographical territory or to specific parents—as the decision rule for restricting residence in a country.”

This may seem like a novel, even revolutionary, argument, but this is the conversation we ought to be having in the twenty-first century. Yet we are further from being able to have this conversation than we were even a week ago. This is the most dangerous and most consistent effect that Trump is having on American culture: he is bending the arc of history backward. He made many promises when he ran for President, but the promise to return to an imaginary past was his biggest. He is keeping it.