On September 11th, 2001, popular perceptions of terrorism were transformed. Although many terrorist activities had occurred earlier, the 9/11 outrages were widely seen as escalating the terrorist danger to an entirely new level. Vast resources were suddenly poured into "the war on terror," sweeping legislation was rushed through Congress, a major new department of the federal government was created, and two foreign countries were invaded and occupied in the name of the struggle against terrorism. There has been no shortage of action. Yet human action is always directed by ideas, and many of the ideas inspiring the current flury of "anti-terrorist" activities are in need of scrutiny and clarification. In Philosophy 9/11, philosophical scholars specializing in terorism-related topics, several of them also officers in the United States military, closely examine some of the basic presuppositions of our response to terrorist attacks.