Foreign fighters have always been integral components of war. With the onset of the 2011 Syrian Civil War and ongoing spill-over effects of intervention in Iraq since 2003, the global foreign fighter problem became exacerbated as the scope of their involvement expanded into a much wider territoriality with significant resources and population base. There is a growing literature on why radicalised individuals from afar countries travel to join an armed movement. Why foreign fighters radicalise and operate the way they do in distant environments gives us insight into why modern conflicts and insurgencies are getting increasingly multi-national, evidenced most clearly first, with Al Qaeda and then, the Islamic State. In this article, we are interested in why people join ISIL is important for three main reasons: (1) Creating a model for radicalization and what condition(s) enable outsiders to fight foreign wars. This is particularly important for understanding volunteerism from countries with high levels of human development, democracy and income equality, which are traditionally offered as remedies against radicalization. (2) Understanding some of the social and economic factors associated with radicalization, that may exacerbate and prolong the effects of radicalization upon foreign fighters’return back to their origin country (the scope of the blowback effect) (3) Establishing a framework for predicting radicalization before it occurs and helping countries take steps in due policy areas to mitigate their nationals joining the ranks of the global foreign fighter pool. We do this by using two extensive datasets of ISIS fighters – the so-called ‘Raqqa Ledger’ and ‘Sinjar Records’, with a detailed breakdown of fighters from origin countries. Then, we use statistical methods to understand whether social, political or economic exclusion is a more important determinant of ISIS recruitment in Muslim and non-Muslim countries. Overall, we find that political exclusion has a greater explanatory value of ISIS recruitment from Muslim countries, whereas social exclusion plays a greater role in non-Muslim countries.