The government argued that the citizenship clause was meant to benefit newly freed slaves and their children rather than “the children of aliens who are temporarily present in the United States or of illegal aliens.” Focusing on the “subject to the jurisdiction thereof” language in the amendment, the government argued that babies born to people unlawfully or temporarily in the country aren’t “subject to the jurisdiction” of the U.S.

But challengers argued that the president’s order attempted a “radical rewriting” of the amendment. They said the matter was already settled by the high court more than 100 years ago, in an 1898 case called Wong Kim Ark, which, they said, “recognized the citizenship of U.S.-born children of virtually all foreign nationals.” The challengers said that precedent “conclusively disposes of the government’s arguments, then and now.” 

On Tuesday, Roberts wrote for the court that children born in the U.S. to parents unlawfully or temporarily present are “subject to the jurisdiction” of the U.S. and are citizens at birth under the citizenship clause.

The chief justice said the holding of Wong Kim Ark “was simple: the Citizenship Clause incorporated the common law and granted citizenship to nearly all children born in the United States. Not surprisingly, then, in the 128 years since, we have repeatedly understood the rule of Wong Kim Ark to guarantee citizenship to all children born in the United States and subject to its power.”

An earlier round of the litigation was before the justices last Supreme Court term, in a case called Trump v. CASA. That case, however, was about the scope of nationwide injunctions that challenged Trump’s order, not the underlying merits of the order. Notably in the CASA case, the administration didn’t press the justices to quickly rule on the order’s merits, even though the administration hasn’t been shy about seeking speedy relief on an array of issues.

Dissenting in CASA last year, Justice Sonia Sotomayor wrote that it was “obvious” why the government wasn’t asking the court to approve the merits of the order. “To get such relief,” she wrote, “the Government would have to show that the Order is likely constitutional, an impossible task in light of the Constitution’s text, history, this Court’s precedents, federal law, and Executive Branch practice.”