War with ISIS

There are somethings I just don't understand. I feel like the views of ISIS are just that - an ideology. I know we can't just declare war on ISIS, because we can't declare war on a state we don't recognize. But regardless, how would we essentially fight them if we could declare war? How could we go to war with an ideology that doesn't play by the rules of war? How can we combat an ideology? I'm not trying to make an argument for anything, these are just my thoughts. I am not an any way qualified to really discuss policy and war strategy but I'm just wondering if anyone can tell me anything.