Shame the treaty's not still in force, otherwise that statement would be part of the supreme law of the land. I wonder if you could make a case for saying that, since the statement relates to the foundation of the country, it endures despite the failure of the treaty. But then, it is a sort of obiter dictum within the treaty. Tough one. It'd probably depend on which judges you got...

Missouri v Holland is an interesting case for states' rights. Doesn't that give the federal government a carte blanche to get around the Xth Amendment any time it wants to? For example, what if the government signed a "gay marriage" treaty with a country that does whatever the US tells it, like Palau? Would that mean that all the states would have to allow and recognise gay marriage, without a constitutional amendment?