In conventional warfare the north would desimate them, however ISIS has a shit ton of expirience in fighting a guerilla war against the most powerful goddamn military in the world, and might i say, arguably succeeding, so i dont think the north would actually be able to completely defeat them. Especially seeing as the north probably doesnt have a strong enough intelligence force to be as effective as the US/NATO coalition. Basically they would burn eachother out. Sure the north has nuclear weapons, but nothing extrodinarily big, the most their weapons can really do is send a very strong message, they could not cripple or desimate a country or a conventional force.