Now that is an interesting question.
Currently, theoretically, the rules are that internal matters are, well, internal (Westphalian sovereignty, basically), and that interactions between sovereign states are subject to international law. This is not ideal, obviously, but I think it is much better than what we do in practice.
Why? Well, the thing is, we tried meddling in internal affairs in an unrestricted fashion and in Europe this experiment led to the thirty years’ war. And note, please, that even then it was over high ideals: after all, should you stand idly by and let those bastards in $OTHERCOUNTRY be led to hell and damnation by those Catholic/Protestant bastards.
Now, this does fall down in certain situations. For instance, ecology is a big one. Natural processes don’t give a toss about national borders. That said, you do get a lot of options under the framework of international law: you can impose sanctions, for instance, and if the UN wasn’t a joke, you could use that to even sanction intervention in certain circumstances, again, according to international law. I can see this law having a ‘eco-disaster’ proviso, certainly.
