The point is...it is difficult to pin this threat down with a definitive name.
Secondly, these monsters are no better than pirates and to "declare war" is to elevate them to the level of legitmate combatants, which they are most definitely NOT. They are vermin, little more than mad dogs infected with the rabies of religious fanaticism. War declaration gives them the "cover" of the Geneva Convention which they do not observe anyway.
Actually they are worse than pirates, pirates really only want your money, well and perhaps your women. These guys just want to kill us. Perhaps better than a declaration of war would be a formal declaration from Congress, and signed by the President, that they are "enemies of the United States". That would have the same effect domestically and would allow us to treat them like pirates were once treated, that is "shoot on sight".