The growth of ISIS brought troubled youth from around the world to support their cause. Radicalized people from Europe, the US, the Middle East and SE Asia rallied to the cause.
Some came for adventure. As some surrender in Syria, many merely came for the money. Others really were radicals.
As ISIS breaks up, many will want to return to the countries they came from. Allowing these radicals back into your society is fraught with danger.
Indonesia is tackling the issue right now and we should watch and learn from how it works. Read more here.