A day after President Donald Trump issued an executive order barring birthright citizenship to children of undocumented residents, the American Civil Liberties Union of New Hampshire sued in the U.S.

Put simply, under Trump’s order, the children of most undocumented immigrants wouldn’t be US citizens, nor would the children of people in the country on student, work, or tourist visas. This is more than a reinterpretation of the 14th Amendment — it’s an attempt to do away with the jus soli principle that undergirds US citizenship.

Until the order, which Trump signed the same day he was inaugurated as the 47th president, the U.S. government has, at least the late 1800s, considered the child of any immigrant born on U.S. soil an automatic citizen, even to a mother in the United States illegally.