“We’re in a new world now,” Solicitor General John Sauer told the U.S. Supreme Court during oral argument this April in the birthright-citizenship case Trump v. Barbara, “where 8 billion people are one plane ride away from having a child who’s a U.S. citizen.” Chief Justice John Roberts quickly shot him down, replying, “Well, it’s a new world,” but it’s “the same Constitution.”Roberts’s quip foreshadowed his opinion on behalf of the Court holding that near-universal birthright citizenship is guaranteed by the text of the Fourteenth Amendment’s citizenship clause. His opinion is a meticulous rendition of U.S. history up to the...