President Trump often trumpets American exceptionalism, but an executive order scheduled to take effect this week seeks to uproot a longstanding policy not found in much of the developed world: granting citizenship to the children of illegal immigrants born on U.S. soil. Under his order, the babies would, instead, inherit the immigration status of their parents. 

Attorneys general from 22 states have already sued in two federal district courts and won preliminary rulings to block what they call the president’s “unquestionably unconstitutional” action. A lawsuit filed by four states in the Western District of Washington claims his action “is contrary to the plain terms of the Fourteenth Amendment’s Citizenship Clause.”  

Legal experts on both sides of the debate agree that the issue likely will be resolved by the Supreme Court. It’s a case raising momentous questions about the meaning of citizenship in a nation founded by immigrants, hinging largely on the legal interpretation of a few words in a Civil War-era amendment to the Constitution.

Trump’s order would align U.S. policy with much of the developed world, including the European Union, Japan, and the world’s two most populous countries, China and India. But it would make the U.S. an outlier on this side of the Atlantic: Almost all of the estimated 33 nations that embrace what’s called birthright citizenship are North and South American nations that accepted waves of mostly European settlers and enslaved people.