This article provides an in-depth analysis of the role of foreign fighter returnees in the attacks by the Islamic State in Iraq and Syria (ISIS) in Western Europe. To do this, it focuses primarily on the operatives and logisticians responsible for the November 2015 attacks in Paris. The threat from foreign fighter returnees remains under-appreciated in expert and policymaking communities. ISIS’s rhetorical emphasis on the caliphate, combined with a series of attacks by lone actors, has made it easy for policymakers to misinterpret ISIS’s true intentions against the West. This case study illustrates that ISIS leaders have been able to plan and execute parallel strategies within the Middle East (Islamic caliphate) and Western Europe (terrorist campaign). Moreover, they have pursued these parallel strategies through using foreign fighters.