Last October, the Yemeni government came to the CIA with a request: Could the agency collect intelligence that might help target the network of a U.S.-born al-Qaida recruiter named Anwar al-Awlaki? What happened next is haunting, in light of subsequent events. The CIA concluded that it could not assist the Yemenis in locating al-Awlaki for a possible capture operation. The primary reason was that the agency lacked specific evidence that he threatened the lives of Americans — which is the threshold for any capture-or-kill operation against a U.S. citizen. The Yemenis also wanted U.S. Special Forces' help in pursuing al-Awlaki;...