We can stop a plot. Get a group of would-be terrorists meeting with each other and our agents can infiltrate it. Get them meeting in Yemen and we can send in the drones. Let North Korea threaten the South and we can threaten them, completely plausibly, with obliteration. Scale is our friend-we know how to detect enemies who go to scale, and we detect so well in these post-9-11 years that it doesn't take much to go to scale.
It's the small stuff that we can't stop. The loners, the solo operators, the guys who march to their own deranged drummers. Be they bombers for some cause or shooters without one, whether we call them terrorists or just mass killers, they're the ones most likely to slip our grasp. You can't penetrate the social networks of the asocial. The unibombers of this world live inside their heads, coming out only in the acts of rage through which we meet them-too late.
As I write, we don't know if the Boston Marathon bombings were the act of one contorted soul or several, but the scale of the attack doesn't suggest a major operation years in the works. This is more like the unsuccessful bombing at Atlanta's Centennial Park during the 1996 Olympics than it is like 9-11-with the signal, mortal difference that yesterday's bombing was more deadly, more bloody, than Atlanta's. In itself, the Boston bombing won't induce dread unless more attacks follow or unless the FBI and the cops link it to a terror network. We don't know enough to justify feeling anxious. We know just enough to feel horror.