Since the 9/11 attacks, conventional wisdom has held that acts of terror on U.S. soil would most likely come from abroad. Much time and effort have been spent on beefing up border security to make sure the next Mohammad Atta never reaches the homeland. Yet the massacre at Fort Hood by Major Nidal Hasan underscored what has long been known in counterterrorism circles: Muslims born and raised in the U.S. might become radicalized and perpetrate acts of terror here at home, and more resources need to be directed inward. Now, a string of arrests by the FBI over the past...