AMERICA’S SUPREME COURT appears unusually uncertain about how to resolve Trump v CASA—a case that could redefine who qualifies as an American citizen and reshape the limits of judicial power. At issue is the 14th Amendment’s promise of citizenship for “all persons born or naturalised” in America. For more than 125 years this has been understood to grant automatic citizenship to almost everyone born on American soil (the children of diplomats and soldiers of invading armies are exceptions). Donald Trump has issued an executive order that claims the clause was never intended to apply to children of undocumented immigrants and temporary visa-holders.