The shortest distance from Russia to America's industrial heart lies across the North Pole and down through the expanse of Canada. Last week, to help guard this vital approach, four more zones of a coast-to-coast air-defense system went into operation along the Canadian-U.S. border.
On the U.S. side, Air Defense Identification Zones (ADIZes) are up to 535 miles deep; on the Canadian side (CADIZes) they are narrower and there is a gap in the prairie belt where the system is not yet in operation. Areas on both sides of the border are rigidly policed. Airmen planning to fly in them must...