Mexico’s ruling class is coming apart at the seams—and the panic is all about one thing: whether to finally get serious about cartel terror with U.S. help. A full-on melee broke out on the floor of Mexico’s Congress of the Union on Thursday after a debate over possible American involvement in crushing the cartels turned into shoves, swings, and a staffer getting knocked to the ground. The brawl laid bare what many on both sides of the border already know: the status quo is unsustainable, and the political left would rather throw punches than confront the criminal armies bleeding Mexico—and...