Incidents like the Snowden affair put my former colleagues in the intelligence community in an impossible position. Yes, the official explanations about the virtues of data-collection efforts can sound self-justifying and vague. But they’re still right. I know firsthand that Gen. Keith Alexander, the NSA director, is telling the truth when he talks about plots that have been preempted and attacks that have been foiled because of intelligence his agency collected. I know because I was on the inside, I have long held security clearances, and I participated in many of the activities he describes.