This paper compares networks of foreign fighters who joined the Islamic State of Iraq and Syria (ISIS) from Europe and the Arabian Peninsula in order to test whether there are differences in their recruitment and how those differences affect the nature of the foreign fighter mobilization. It is the first study to compare different networks of foreign fighters that joined the same group in the same conflict at the same period of time. This study finds that foreign fighter recruitment resembles an efficiency-secrecy tradeoff: in places where recruitment needs to be hidden from legal scrutiny, recruitment networks are decentralized; composed of small and more local recruitment cells. These cells can operate more secretly and the group as a whole is more resilient to disruption. In exchange, it is hard for the group to attract large numbers of recruits. Whereas in places where recruitment could occur more freely, recruitment networks are more hierarchical; comprised of a larger number of recruits with more geographically diverse connections. The hierarchical design of their recruitment networks may be easier to disrupt, but it also helps the group efficiently recruit more followers if left undisturbed. This study concludes that the ISIS foreign fighter recruitment process varied significantly. Researchers and policymakers focused on recruitment and radicalization should therefore carefully frame their results or policies based on the different types of recruitment processes and the various social, political, and legal contexts where their work takes place.