The key is that once a treaty is ratified by a 2/3s vote in the senate, and the president signs it, it becomes "law," and the US government is obligated to obey it. While any law or treaty passed cannot superceed the Constitution, a treaty can superceed federal, state, and local laws.
Mark
And we even have "treaty enforcers". That is one of the duties of the US Coast Guard, as part of the department of Homeland Security.