Are the borders of country 5,000 miles away more sacrosanct and more worth taking existential risks than our own airspace and southern border? One of the strangest things about the American response to Ukraine has been the willingness of the Left and the establishment Right to discount completely that the war is heading toward a rendezvous with ever-deadlier weapons and staggering fatalities—even as we witness increasing nuclear threats from a weakened and adrift Vladimir Putin. They insist that Putin is merely saber-rattling. And he might be. Supposedly, in his diminished and discredited state, Putin would not dare to set off...