People will need to realize how the second world war was won.
The Japanese did horrible things, the SS did horrible things. In that regards, ISIL is not that much different.
Actually, ISIL make the same error as the Japanese. The Japanese thought that they could terrorise the opposing armies by acting brutally and barbaric. That did not work out for them (look what happened on Guadalcanal).
The main problem is that current opposing political forces in the Middle East will have to work together to crush ISIL.