Where is that written? I would expect naming a transnational group would be acceptable and sufficient to delineate who we were declaring war upon.
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.