David Sterman points out another reason why the threat of homegrown jihadis fails to live up to our outsized fears of it:
Once in an American city, an extremist must still acquire weapons. And if he plans to conduct a large-scale strategic attack (rather than a lone wolf-type shooting), he must also connect with others, engage in planning and surveillance activity, and finally prepare and carry out the attack. All of these steps are constrained by the willingness and ability of local Muslim and non-Muslim communities to report extremist and suspicious activity, as well as by the domestic efforts of law enforcement. …
None of this is to say that Jihadist groups in Syria should be allowed to fester and develop the capability to conduct attacks in the United States, or that it is impossible that a returning Syrian foreign fighter will evade the layered defenses that protect the American homeland. That Abu Salha was able to return undetected to the United States after participating in Jihadist training should concern law enforcement. The layered defense system may need reinforcement to deal with new challenges, but the constraints it imposes upon jihadist activity ought not be obscured, particularly when making the case that the threat posed by foreign fighters calls for military action. Doing so does a great disservice to the admirable efforts of Muslim communities, local and federal law enforcement, and American citizens in confronting Jihadist extremism at home.