Islamic State: What happened to all the foreign fighters?

An estimated 40,000 people traveled from around the world to take up arms for the Islamic State group as it occupied territory in Syria and Iraq and declared a caliphate in 2014. How many have gone elsewhere to fight?" said Seth Jones, director of the International Security and Defense Policy Center at the Rand Corporation. International counterterror groups are putting huge efforts into answering those questions, working hard to name, count and track IS foreign fighters.