'So history teaches that a military "solution" will fail.'

What history teaches is that this type of military solution fails. Unless allied countries are prepared to send soldiers who in turn are told to take no prisoners and willing to carry out that order, the second way is diplomacy of a less violent nature. That will likely fail unless some country is prepared to offer up a pile of land for the Daesh fighters to live in. The third option is to ignore them and I doubt that will work because they hate being ignored. Helluva puzzle.