Whether or not Islamist terrorists prove capable of executing another attack on the United States on the scale of the strikes of September 11, 2001, we would be foolish to assume they can’t. The desire remains, while the bloodlust and the passion have only intensified. The willingness to sacrifice their lives to do us harm is indisputable. We are more vigilant and—somewhat—less willfully naïve, and grand attacks on the homeland are harder to stage today, but the price of deterrence is high in economic costs, resources, and diverted energies. Even failed attacks trigger expensive, long-term responses.