When President Donald Trump’s birthright-citizenship order reached the Supreme Court, the smart money said it would lose.

The question was whether the justices would take the narrow path Justice Brett Kavanaugh ultimately mapped out in his partial dissent, or constitutionalize the whole issue.

They chose the latter.

In Trump v. Barbara, a five-justice majority held that the 14th Amendment itself guarantees citizenship to children born here to parents here illegally or only temporarily.

That result will be hailed as a vindication of the post-Civil War promise of equal citizenship.

But the opinion is more sweeping than it needed to be — and, on the originalist question, less inevitable than its defenders pretend.

For once, I agree with Kavanaugh’s middle-of-the-road approach.

He would’ve invalidated Trump’s order because it conflicts with a 1940 federal law that was eventually folded into the Immigration and Nationality Act.

That would’ve been a perfectly respectable way to say, “Mr. President, you can’t do this on your own. If the elected branches want to revisit these rules, Congress itself has to speak.”

Kavanaugh’s route wouldn’t have settled any historical debate, but it’s what courts should do when the statutory answer is relatively straightforward and the constitutional question isn’t.

Reasonable originalists can disagree over whether “subject to the jurisdiction” — the key phrase in the 14th Amendment — just means being subject to American law, or a more complete allegiance tied to domicile and political membership.

Anyone who says the administration’s position was a Trumpian fever dream hasn’t spent much time with the Reconstruction debates, early executive practice or Wong Kim Ark (an important but not dispositive 1898 case involving the child of resident aliens).

That doesn’t mean Trump’s order was automatically valid: Executive orders aren’t amendments to the US Code, much less to the Constitution.

The administration tried to solve by presidential pen what only law — if not constitutional law — could.

But the majority went further than necessary and turned a modern policy dilemma into a constitutional command.

Justice Samuel Alito’s dissent exposes the problem, even for those who don’t agree with his bottom line.

“The Fourteenth Amendment dictates who must be a citizen,” he wrote, “but it does not address who may be a citizen by Act of Congress.”

In other words, the 14th Amendment sets a floor, not a ceiling.

Congress has long granted citizenship to people not constitutionally entitled to it, including children born abroad to American parents.

The harder question was whether Congress could create prospective exceptions for children of non-citizens with no permanent tie to this country.

After Barbara, the answer is no.

Trump says he wants Congress to fix this, but because the court reached the constitutional question, Congress can’t simply redefine “subject to the jurisdiction.”

It can of course propose a constitutional amendment, but that isn’t a legislative fix in any ordinary sense.

Short of that, the path Kavanaugh would’ve left open has been blocked.

That matters, because the modern problem is real.

Birth tourism was unknown and practically inconceivable in 1868: There were no package deals for foreign nationals to fly into a five-star birthing center, obtain an American passport for Junior and go home.

Illegal immigration likewise wasn’t on the minds of the 39th Congress.

Originalism isn’t a séance, but it does require applying constitutional meaning to circumstances the ratifiers never anticipated.

The dissenters were right that new conditions demand attention to the principles behind old exceptions — and that perhaps, as Justice Neil Gorsuch suggested, the kids of longtime illegal residents could be grandfathered in, on a case-by-case basis.

The majority instead treated a longstanding practice as legally definitive.

That may bring administrative certainty, but it also freezes a controversial citizenship regime into the Constitution.

So where does that leave Trump? Not where he wants to be, but not nowhere.

The federal government still controls visas, admissions, removals and border security.

It can tighten vetting to reduce birth-tourism abuse, further protect the border and remove overstays and illegal entrants.

These are all unquestionably lawful tools.

Regardless, the court’s decision won’t make the birthright-citizenship debate disappear.

It will make it harder, and even more political.

Trump lost the constitutional battle. Now he has to fight the enforcement war.

Ilya Shapiro is director of constitutional studies at the Manhattan Institute and author of the Shapiro’s Gavel newsletter.