When historians look back on this moment in American history, the date that may stand out to them is not Sept. 10, when a gunman took the conservative activist Charlie Kirk’s life. They may focus instead on Sept. 15, when the vice president of the United States promised that the federal government would use the assassination to begin cracking down on leftist organizations. On that day, while guest hosting “The Charlie Kirk Show,” a podcast, JD Vance pledged that the Trump administration would “go after the NGO network that foments, facilitates and engages in violence,” though he provided no evidence...