President Trump promised Monday night that he would use an executive order to end birthright citizenship for children of noncitizens. This proposal and his arguments for it highlight how the “America First” nationalism of the President compromises American exceptionalism and makes America a smaller, weaker and more divided country.

The President’s primary argument for abandoning the historic policy of the United States, codified in the Fourteenth Amendment to the Constitution, is that America is “the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen.” This is false. Canada and about 30 other countries have followed America’s lead on this issue, granting citizenship at birth to anyone born in their country.

But the fact that the President believes that this is a valid reason for rewriting the Constitution by executive fiat illustrates something about his nationalist philosophy. Rather than seeking to keep America exceptional — to use Ronald Reagan’s iconic phrase, “the City on the Hill” — he would seek to make America more like other countries, ones without a Constitution at all.

The President’s argument is precisely the same as arguing that because America is the only country in the world with a First Amendment or Second Amendment, it should abolish the rights to free speech or firearm ownership. No one who values the unique character of the U.S. Constitution would accept this argument.

But Trump’s nationalism isn’t one built around the American experience. Instead, he seeks to make America more like ethnic nation-states in Europe where citizenship is built on the right bloodline.

Being American isn’t about having the right parents. It’s grounded in the experience of the individual, not their ancestry.

Trump’s secondary argument was that birthright citizenship is a “ridiculous” policy because a “baby is essentially a citizen of the United States for 85 years with all those benefits.” As for the benefits that they receive, the National Academy of Sciences (NAS) has found that second-generation Americans are the most fiscally positive of any U.S. residents, producing far more in value than they get back.

For Trump, millions of Americans who are the children or grandchildren of noncitizens have basically stolen their U.S. citizenship and so should not be — and, if he got his way, would not be — Americans. According to the NAS, “the citizenship status of 37.1 million second-generation Americans living in the country (about 12% of the country’s population), and perhaps many millions more in the third and higher generations would be up for debate.”

This policy fits into Trump’s goal of dramatically cutting legal immigration. If he had his way, at least 60% of all legal immigrants since 1965 — 23 million people — would never have come to the United States at all. America would likely have 52 million fewer people. Indeed, without immigrants and their children, America’s population would soon begin to decline.

Beyond the benefits of making a stronger and more powerful country, birthright citizenship fuels assimilation. Because every child born in America knows that they are a U.S. citizen, they are more likely to adopt American values.

This brings us to the final dimension of the President’s nationalism: Rather than a nationalism that unifies Americans around a common set of values — like the nationalism of the founders — the President’s nationalism seeks to divide them. Ending birthright citizenship would quickly transform America into two groups: “real” Americans who have the right bloodlines and “fake” Americans who don’t.

The government would quickly need bureaucrats to investigate people’s backgrounds to make sure that they were born to the “right” parents. It would turn Americans against each other.