In this paper we first put ISIS volunteers in context by considering other examples of American citizens fighting in someone else's war. Next we consider poll results indicating that many U.S. Muslims perceive a war on Islam and prejudice against Muslims; at least ten percent of younger U.S. Muslims justify suicide attacks in defense of Islam. Against this background it is perhaps surprising that only a few hundred U.S. Muslims have volunteered to fight in Syria. In the absence of accurate data about U.S. volunteers, we review what has been learned about the thousands of European volunteers for ISIS, many of whom seem to be pushed to action by individual-level mechanisms described by McCauley and Moskalenko in 2011. Finally, we raise doubt about current efforts to criminalize and block would-be volunteers.