The problem in stopping terrorist travel to the U.S. is not airport screening per se. Trying to turn every airport into another Maginot Line or Fort Knox is going to fail sooner or later. Instead, the best way to stop terrorist plots is to frustrate them before they get started--thwarting the 2006 London-based conspiracy to smuggle liquid explosives on U.S.-bound international flights is a good example of just how effective such a preemptive strategy can be. Until the terrorists are rooted out, however, the free nations under threat from global terrorism have to do a better job at thwarting terrorist...