It was instantly clear after Sept. 11, 2001, that our security agencies knew little about al Qaeda's inner workings, could not detect its operatives' entry into the country, nor predict where it might strike next.

Suppose an al Qaeda cell in New York, Chicago or Los Angeles was planning a second attack using small arms, conventional explosives or even biological, chemical or nuclear weapons. Our intelligence and law enforcement agencies faced a near impossible task locating them. Now suppose the National Security Agency (NSA),...