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The US already had its out in the 90s on this, regarding international law. Long before Bush/Cheney raped the Constitution, Clinton and Bill Richardson carved out American exemption from international law. Just follow the whole International Criminal Court stuff from that time, the establishment of "precedent" for American lawlessness -- the US opted out of the Court for fear of "politically-motivated charges" being levied against American operatives.
Actually, the carving out, in terms of actual signatures and ratifications, can be traced to the 1980's, and Douglas Feith lobbying the Reagan Administration, successfully, to send the 1977 Additional Protocols to the Geneva Conventions to the Congress with a recommendation of no ratification. Yep, that'd be the same Douglas Feith.
And during Clinton, a lot of pushing for peeling away from international law came from a group that was, at the time, referred to as the 'New Sovereigntists', who disputed whether common or customary international law was in any way binding on the U.S. and opposed the Vienna Convention on the Law of Treaties. That group was composed of some other, little known, lawyers: Jack Goldsmith, John Yoo, John Bolton,...hmmmm.
So, like you, I used to say this had old roots, but, unlike you, I put them in the 1980s with people like Feith.
But then came this interview with International Law scholar Mary Ellen O'Connell by Scott Horton:
http://www.harpers.org/archive/2008/12/hbc-90003966
She most clearly pegs it, with facts to back it up, on the 1960's, and the foreign policy theories of a guy named Hans Morgenthau.
[Carl] Schmitt’s ideas did indeed come to be reflected in the foreign and legal policy of the Bush Administration. Leo Stauss’s dismissal of international law and his support for the strong-man leader came to the United States primarily through Hans Morgenthau. Morgenthau’s influence in U.S. foreign policy from the 1960s forward has been extraordinary. By then, the very highest U.S. officials were no longer speaking the language of international law and institutions but of projecting American power. U.S. legal thinkers, of course, took note, and, by the 1960s were also focused on the Civil Rights movement and the Women’s Movement. Those movements were primarily concerned with constitutional law, and constitutional law became the prestige topic in U.S. law schools—some top schools no longer even taught international law. The result was that by the 1990s, international law was increasingly being mischaracterized as weak, unimportant or even dangerous—nothing that should fetter the superior American state. And Bush officials were able to find in the Constitution support for a strong-man leader above the law in wartime.
I recommend her interview, if you want to really see how deep these roots go.