The cold-blooded murder of two New York City policemen as they sat in their car is not only an outrage but also a wake-up call. It shows, in the most painful way, the high cost of having demagogues, politicians, mobs and the media constantly taking cheap shots at the police. Those cheap shots are in fact very expensive shots, not only to the police themselves but to the whole society. Someone once said that civilization is a thin crust over a volcano. The police are part of that thin crust. We have seen before our own eyes, first in Ferguson,...