“Decimate” is the wrong term as it means to cull 1 out of 10, at random. Erradicate, Annihilate, or Obliterate would be better word choices. Or... kill all of them, their wives, their brothers and sisters, son and daughters... hell, take out their goats too... total war. Make the whole Fertile Crescent sterilized glass.
Bomb the terrorists, not the families. And arms the Iraqi, Kurds and Syrian rebels to defeat ISIS.