Terrorist Displacement: Foreign Fighter Returnees and Extremist Plots in the United States, 1980-2016

Fighters returning to their home countries from foreign conflicts are a central security concern for many national governments. While analysts have made dire warnings about the dangers posed by returnees, little empirical research has analyzed whether they are a greater threat to national security than extremists who do not fight abroad. To address this need, we examine a supplement to the Profiles of Individual Radicalization in the United States (PIRUS) dataset that includes information on the travel and plot activities of 468 U.S. Islamist extremists from 1980-2016, including 57 who returned to the U.S. from foreign conflicts. Using mixed methods, we show that while returnees pose a moderate threat, individuals who do not travel or who are unsuccessful in their attempts to do so are more likely to carry out successful terrorist attacks in the U.S. Furthermore, our results suggest that terrorist plots can be the result of a displacement effect that occurs when security measures are put in place to prevent individuals from traveling to fight abroad. More specifically, during the period we reviewed, individuals who aspired to join foreign conflicts but were prevented from doing so had the highest rate of involvement in U.S. terrorism plots.