International Law’s Erie Moment

The episode put the question starkly: Who fills the gaps in international law and how? A series of tribunals operating under Chapter 11 of the North American Free Trade Agreement (NAFTA) had adopted broader interpretations of vague treaty language than those recommended by the state parties. In response, government ministers from the three state parties, Mexico, Canada, and the United States, operating through the Free Trade Commission (FTC) established by the treaty, adopted “Notes of Interpretation” clarifying their view of the treaty’s meaning. International tribunals are generally tasked with examining state practice, either to recognize rules of customary international law that state practice may evidence or to discover from subsequent practice the intended meaning of treaty provisions. But when a tribunal’s own interpretations conflict with state practice, who wins? Is state practice contrary to a tribunal’s decision a breach of international law or a rebuke of the tribunal’s view? In the case of NAFTA, the general instruction of the Vienna Convention on the Law of Treaties to look to “subsequent agreement” and “subsequent practice” in interpreting treaty provisions was paired with a specific recognition of the FTC’s authority to “resolve disputes that may arise regarding [the] interpretation or application” of NAFTA. Nonetheless, NAFTA tribunals were split on exactly how to regard the “Notes of Interpretation” and how much respect they should be due. While for some, the Notes provided clear evidence of the parties’ intent, for others, they were an improper attempt to subvert due process and the rule of law.