'We cannot and will not be passive in the face of such violence,' President Barack Obama said in January 2011, after a deranged gunman shot Congresswoman Gabrielle Giffords and 18 others, killing six. "We should be willing to challenge old assumptions in order to lessen the prospects of such violence in the future." He called for a "national conversation" about "everything from the merits of gun-safety laws to the adequacy of our mental-health system," and he asked that it be conducted with civility. It was a terrific speech, perhaps the best of his presidency. And then ... nothing.
There has been...