Imagine the following dilemma facing an American president one day in the future (when, we hope, our real-time intelligence will have reached a high degree of accuracy, precision and timeliness): Within the past hour, a terrorist organization, known to have acquired several nuclear weapons, has been observed by a U.S. imaging system loading the weapons onto vehicles and preparing to leave for an unknown destination. A delay of even an hour or two in launching a U.S. strike on that location could mean the group would depart, contact might be lost, and the weapons would be smuggled into the United...