Belgium has seen more foreign fighters leave for the ISIS ‘caliphate’ than any other western European country in per capita figures. But among Belgian foreign fighters, as well as among Belgian terrorists who did not travel abroad, a stunning majority have been Belgian-Moroccan. What factors can explain this phenomenon? Both groups live in relatively similar socio-economic settings, experience stigmatization and discrimination, and both are, relative to broader Belgian society, poorly integrated. However, in investigating this question, an interesting caveat emerges vis-à-vis the ‘integration deficit’ theory: Belgian-Turks, who are found to be comparatively less integrated than the Belgian-Moroccans, are underrepresented in extremist milieus. This paper argues that the insularity of the Belgian-Turkish population, a phenomenon not found to the same extent among Belgian-Moroccans despite also being poorly integrated (with some outliers), has protected the population from negative environmental elements, both criminal and extremist. That there are seen to be local factors that contribute to a ‘vulnerability’ to the pull of crime and extremism, means that such conditions are likely to remain even if a group like ISIS fades into irrelevance.