Caveat: The Supremacy Clause and Judicial Activism

Ever since the U.S. Supreme Court's decision in Medellin v. Texas (earlier this year), I have felt deeply troubled.  The so-called "supremacy clause" of the U.S. Constitution states:  "all Treaties made under the authority of the United States, shall be the supreme Law of the land."  But the court held that unless the Congress passes specific laws implementing treaties, treaties are not binding domestic laws.  How is this interpretation in any way in line with constitutional intent?

I'm not trained in law, but the supremacy clause, and our governments' disregard for it, has always troubled me.  I feel strongly that the "founding fathers'" intent was in line with something U.S Grant subsequently envisioned: 

I believe at some future day, the nations of the earth will agree on some sort of congress which will take cognizance of international questions of difficulty and whose decisions will be as binding as the decisions of the Supreme Court are upon us.

Essentially, I think that the Supremacy Clause must have specifically had in mind the sorts of binding supranational treaties we now see with organizations of states such as the U.N. or the E.U.  I don't think that such supranational polities were in any way beyond the conception of the founders of the U.S., given that it seems very likely they viewed their own project as a supranational rather than national project – there was nothing subtle or requiring interpretation in the name they chose:  United States of America.  They expected more states to join, they thought they were already supranational, and they were not trying to build a nation, but rather replace it, as a concept, with something new.  In this sense, the E.U. as it stands today is perhaps closer to their conception than the contemporary U.S. 

That's just my opinion.  So… whatever.  I had a bad day today at work, so I decided to rant on about something where I'm both uninformed and singularly unempowered.