Americans continue to wonder whether the terrorist rampage at Fort Hood could have been prevented. What did government agencies know about Nidal Malik Hasan, and when did they know it? And how deep were Hasan’s jihadist connections overseas? In this post-9/11 world, it seems hard to believe. Details are emerging that U.S. government agencies knew about Hasan’s contacts with Islamic radicals linked to al Qaeda -- and concluded that those contacts did not pose a threat to America’s national security. For months, federal investigators monitored Hasan’s e-mail correspondence with Anwar al-Awalki, an al Qaeda recruiter living in Yemen who has...