In only the last several years, corporate CEOs and conservative influencers have been killed in brazen attacks. Republican justices, presidents, and their staffs have been marked for threats, assaults, and even death. Small-cell terrorist organizations have executed sophisticated attacks on law enforcement. And much of it has been excused, even sometimes encouraged, by an intellectual ecosystem on the left that is, even now, incubating more political violence. In "Blood & Progress," from which this excerpt is drawn, the National Review’s Noah Rothman documents how and why left-wing...