International law?

One more thread and then I’ll relax for awhile (unless something catches my interest, of course).

Zachriel said at one point that our invasion of Iraq offended all nations who support international law.   It is arguable, of course, that the invasion did not violate international law and was merely an effort to correct violations of that law as established by UN resolutions (though I question whether individual member states can take that job upon themselves or whether only the UN should be permitted to enforce UN resolutions). 

In any event, it raises an interesting question.  What is international law?  Indeed, is there such a thing?  How is it created?  How, and by whom, is it properly enforced?  From what does it derive its authority?  What are its limits?  To what extent can/should/must nation states yield their sovereignty to some supra-national body?

What do you think?