The biggest lesson of the 9/11 attacks was to wake the nation up to its institutional counterterrorism failings. Just as those failings missed what would now be quite easily identified footprints of a rising terrorist conspiracy, they would also have enabled a far more lethal strike on the American homeland.

We are lucky that al Qaeda did not believe that a strike involving nuclear, biological, or chemical weapons was possible. We are also lucky that al Qaeda chose to avoid targeting children, in contrast with Chechen Salafi-Jihadist organizations and disrupted Islamic State plots in Europe. Instead, Osama bin Laden’s group saw airliners as desirable weapons for two reasons. First, because they could simplify and amplify the use of explosive materials in creating lethal effect. Second, because they would cut to the heart of the U.S. economy and the American social expectation of easy and safe mobility.