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Imagine you are a law-abiding green-card holder returning to the United States after visiting your parents abroad. At the border, you expect an easy entry; after all, as a lawful permanent resident, you generally do not need to seek admission anew each time you return from travel abroad. But when you present your documents to a border officer, he accuses you of committing a crime. You deny the accusation, because it is untrue, but the officer doesn’t care. He confiscates your green card and refuses to recognize that you are a lawful resident who has already been admitted to the U.S. Now he holds your fate in his hands: He can send you into detention or turn you away altogether. In the best-case scenario, he may “parole” you into the country as an “arriving alien”—forcing you into a legal limbo that makes you far more vulnerable to deportation and could stretch on indefinitely. All without the officer offering persuasive evidence that you committed a crime.

On Tuesday, the Supreme Court blessed this Kafkaesque nightmare by a 6–3 vote along the usual partisan lines. Justice Clarence Thomas’ majority opinion in Blanche v. Lau declared that officers do not need “clear and convincing evidence” that a green-card holder committed “a crime of moral turpitude” before treating them as an “applicant for admission” who may be denied entry, detained, or (at best) conditionally allowed back in on parole. Indeed, Thomas expressly declined to say what, if any, burden the government bears at the border—an ambiguity that the Trump administration will surely exploit to throw green-card holders into deportation limbo. As Justice Ketanji Brown Jackson explained in dissent, “the court has now handed the government a massive blank check” to hollow out “the benefits and security that come with having a green card.”

The origins of Tuesday’s case illustrate its stakes. In 2012, a........

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