Americans are now learning what citizens of Indonesia, Saudi Arabia, Spain, the United Kingdom, and other foreign countries have long known: that some of our own can and will go to great lengths to kill their fellow citizens. Ramy Zamzam and four friends from the Washington, D.C., suburbs were detained in Pakistan in a police raid on a house allegedly tied to a militant group earlier this week. One of the men had recorded a video filled with images of war and declarations that young Muslims must act. The five Americans, students in their 20s, are now being questioned by...