Killer drones have conquered the skies. Can we ever be safe again?

Consider a small military drone, electrically powered, commercially sourced, guided by GPS and a cellular link and the patience of whoever is holding the controller. The drone costs around $500. The missile sent to destroy it costs $2 million. This is, at the moment, the defining arithmetic of modern air defense, and the people responsible for solving it are very aware that they have not yet done so.We have been in similar situations before. When the first aircraft appeared over battlefields in...