http://news.independent.co.uk/world/politics/story.jsp?story=562237Since we're part of the UN, and all treaties are the highest law of the land in the United States, you are incorrect (which is understandable, given the amount of insanity going on in the news these days).
Also, soldiers are not required to wait around for a legal body to declare something illegal to avoid committing war crimes. They are duty-bound to not commit such actions as those that went on at Abu Ghraib, legal declaration or no.