Over the last several weeks, ISIS has been systematically losing territory. Its last stronghold in Iraq, the city of Hawija, was liberated in early October. A few weeks later, ISIS' de-facto capitol in Raqaa, Syria fell to US-backed forces. ISIS no longer controls any major city in the region.   With the group mostly defeated on the ground, the international community is starting to think through some difficult and fraught questions of how best to bring ISIS to justice for genocide, war crimes and crimes against humanity committed during their brutal reign.    On the line with me to discuss some of the options that the international community is weighing, and also some of the key obstacles for bringing to justice those who committed atrocity crimes in Iraq and Syria, is Dr. Zachary D. Kaufman.     Zachary D. Kaufman is a senior fellow at the Harvard Kennedy School of Government and teaching at Stanford Law school -- he is also, like me, a Humanity in Action senior fellow.