Well, I don't know. I don't think anyone does for sure -- which ought to be a striking realization after the billions and billions spent, eh? After the millions of hours and millions of inconveniences suffered at the hands of our airline security "protectors," eh? After the war-deaths of 3,000 or so really, really good people and thousands of serious injuries suffered by the same sorts of good people who happen to be soldiers.
If you're up to it, and care to challenge what you think you might know, spend the 10 or 15 minutes necessary to John Mueller's essay over at CATO Unbound.