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The “Harmon Doctrine” is perhaps the most notorious theory in all of international natural resources law. Based upon an opinion of Attorney General Judson Harmon issued a hundred years ago, the doctrine holds that a country is absolutely sovereign over the portion of an international watercourse within its borders. Thus that country would be free to divert all of the water from an international watercourse, leaving none for downstream states. This article looks closely at the Harmon Doctrine in historical context. An examination of the conduct of the United States during the dispute with Mexico over the Rio Grande that produced the Doctrine, as well as other contemporaneous and subsequent practice, demonstrates that the United States never actually followed the Doctrine in its practice. It is therefore highly questionable whether this doctrine is, or ever was, a par of international law.