In the wake of the San Bernardino attacks, Americans must confront the undeniable reality of homegrown Islamist terrorism. We must also confront how little we have learned since 9/11 about Islam and about the Muslims who are our fellow citizens. In recent days our public officials—at least the serious ones—have not been able to articulate anything more cogent than “If you see something, say something,” a tired slogan originally developed by the New York City Metropolitan Transportation Authority in 2002 and officially adopted by the Department of Homeland Security more than five years ago.