"The Islamic State’s project to establish a proto-state and expand its domain across a broad swath of Iraq and Syria attracted tens of thousands of foreign fighters from at least 80 countries..."

"Some security experts say it is crucial for countries to repatriate and prosecute their nationals if the jihadist movement is to be prevented from rising again. They add that due process also requires this. But the difficulties are many. Some governments don’t even know how many of their nationals are being held, who they are or how deeply they were involved in Islamic State atrocities. Many countries lack the laws to prosecute alleged fighters, and even if the laws are on the books, it is often unclear whether evidence from the battlefield would hold up in court. If they are convicted, they could radicalize others in custody and then be released after sentences as short as three years, leaving already overburdened police to keep unrepentant militants from turning to violence again. Western European countries, in particular, have been resistant to bringing the prisoners home, citing obstacles ranging from national security to domestic politics...."

From "After the Caliphate: Disarmed but not defused/The defeat of the ISIS caliphate left this Moroccan militant and about 2,000 other suspected foreign fighters detained in northeastern Syria. Will they pose a greater threat there or back in their home countries?" (WaPo).