I put up a couple of posts about the death camps in Nazi Germany recently. At the time I knew I was making an analogy. The only thing that made the Nazis stop was enough people with weapons and the will to use them. We demanded unconditional surrender and then we did what ever was necessary to back up that demand. It wasn't necessarily honorable or kind or measured. We brought enough death and destruction that finally the remaining survivors surrendered.
ISIS took a pilot that they had captured, put him in a cage, doused him with gasoline and set him on fire. This is not conjecture. They videotaped it.
We can do nothing and sit back and watch more of this. We've done that before. The Nazis took power in Germany in 1933 and it took us until 1945 to sort them out. But one thing is for certain, this will not stop until it is stopped by force.