On Thursday, the Supreme Court unanimously ruled that the government of Mexico may not continue its lawsuit seeking to hold firearms manufacturers and a firearms distributor civilly accountable for their role in causing cartel-driven gun violence in Mexico. Having taken the case at an unusually early stage in the litigation, and so working from an undeveloped factual record, all nine justices agreed that Mexico’s current complaint does not even satisfactorily allege that the defendants have aided and abetted U.S. dealers who illegally sell guns to traffickers who then get them to the cartels in Mexico. What’s worse, Justices Clarence Thomas...