After major terrorist attacks hit Brussels and Paris, Europe is still reeling. These atrocities prompted a heated dispute between two noted French scholars of political Islam, Gilles Kepel and Olivier Roy, over why a handful of European Muslims become terrorists.

Their disagreement isn’t just an intellectual squabble. It strikes to the heart of how the West understands violent Islamist extremism — and what the appropriate policy responses might be.

European terrorists are mostly homegrown

Most of the assailants in the recent Brussels and Paris attacks were raised in either France or Belgium. As were those who carried out the January 2015 assault on the Charlie Hebdo magazine in Paris. Why did these people turn against their own countrymen — and who or what is to blame?