The United States takes a different view concerning the relationship between international and domestic law than many other nations, particularly in Europe. Unlike nations which view international agreements as always superseding national law, the American view is that international agreements become part of the body of U.S. federal law. As a result, Congress can modify or repeal treaties by subsequent legislative action, even if this amounts to a violation of the treaty under international law. The most recent changes will be enforced by U.S. courts entirely independently of whether the international community still considers the old treaty obligations binding upon the U.S. Additionally, an international agreement that is inconsistent with the U.S. Constitution is void under domestic U.S. law, the same as any other federal law in conflict with the Constitution, and the Supreme Court could rule a treaty provision to be unconstitutional and void under domestic law, although it has never done so. The constitutional constraints are stronger in the case of CEA and executive agreements, which cannot override the laws of state governments.

The U.S. is not a party to the Vienna Convention. However, the State Department has nonetheless taken the position that it is still binding, in that the Convention represents established customary law. The U.S. habitually includes in treaty negotiations the reservation that it will assume no obligations that are in violation of the U.S. Constitution. However, the Vienna Convention provides that states are not excused from their treaty obligations on the grounds that they violate the state's constitution, unless the violation is manifestly obvious at the time of contracting the treaty. So for instance, if the US Supreme Court found that a treaty violated the US constitution, it would no longer be binding on the US under US law; but it would still be binding on the US under international law, unless its unconstitutionality was manifestly obvious to the other states at the time the treaty was contracted. It has also been argued by the foreign governments (especially European) and by international human rights advocates that many of these US reservations are both so vague and broad as to be invalid. They also are invalid as being in violation of the Vienna Convention provisions referenced earlier. (wiki)