On Wednesday, the Supreme Court will hear oral arguments on the constitutionality of President Trump’s executive order aimed at ending birthright citizenship as we have known it. The court’s eventual opinion in the case, Trump v. Barbara, will almost certainly hinge on how the justices interpret the 14th Amendment’s Citizenship Clause, which says: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside.” The court will probably also respond to the first words of the president’s March 19 brief, which asserts that...