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Sometimes the most consequential shifts in legal power do not arrive through sweeping constitutional pronouncements. Sometimes they arrive disguised as an argument over a single preposition. To explain why the government prevailed in Thursday’s asylum case, Mullin v. Al Otro Lado, Justice Samuel Alito, writing for the court’s majority, reached for a football field, a front porch, and a mailbox.

A running back tackled at the 1-yard line has not reached the end zone. A guest standing outside the front door has not arrived in the house. A letter still in the mail carrier’s hand has not arrived in the mailbox. Likewise, Alito writes, an asylum-seeker standing in Mexico has not “arrived in the United States,” even if an American border officer is the only thing preventing that final step. The ordinary meaning of the statute, the court concludes, is straightforward: Someone has not arrived until he is inside.

As exercises in ordinary language, those analogies are difficult to quarrel with. But this case should be understood as something more than a dispute over grammar. For its part, the majority insists it is answering a narrow question of statutory interpretation. The Immigration and Nationality Act, it explains, requires immigration officials to inspect a noncitizen who “arrives in the United States” and permits that person to apply for asylum. The question before the court was whether someone standing at an official port of entry on the Mexican side of the border has already “arrived,” or whether arrival occurs only after physically crossing onto American soil. The court chose the latter. Under that reading, until an asylum-seeker takes that final step, the government’s statutory obligation to inspect the person and begin the asylum process never arises.

Justice Alito’s opinion is a classic exercise in textualism. He works methodically through dictionary definitions, neighboring provisions of the Immigration and Nationality Act, Congress’ decision in........

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