The law, by definition is a rule backed up by the use of force, specifically state-sanctioned violence. If you write a law but do not have the ability to use a sufficient amount violence to enforce it when needed, you don't have a law at all, you just have a suggestion around how you'd like people and countries to behave.
The only way you could ever have anything resembling "international law", would be to have some sort of global military or police force capable of exerting enough violence to ensure that the law is followed, and I'm not even sure how such a thing would work.
