That raises an interesting question of jurisdiction and what it means for something to be "illegal". In the US its illegal to sell marijuana, but in some nations it's not. Yet I don't think we'd say that the citizens of those nations are breaking US law when they sell marijuana to one another in their own country. If the US hasn't signed up to submit itself to the legal authority of the ICC, I don't think it makes any sense to talk about them doing something "illegal", since that implies that are part of that legal system. A nation's acts might be imperialistic, aggressive, dangerous and exploitive (and perhaps rightly opposed for those reasons)...but "illegal" seems a bit of a stretch. It requires an explanation of which legal code is being broken and why the given jurisdiction should extend to the nation in question. IMO, the term "international law", and the legal terminology associated with it, implies a connection with national laws which is tentative at best.