In Medellín v. Texas, a Texas death penalty case, the United States Supreme Court decided that it could not enforce what it acknowledged to be an international legal obligation to comply with the Avena judgment of the International Court of Justice. The Supreme Court's judgment in Medellín has put our understanding of the domestic treatment of US treaty law in a state of flux. Under the Supremacy Clause of the US Constitution, treaties are the supreme law of the land: binding, equivalent to federal statutes and enforceable by judges. After Medellín, treaties may not necessarily be enforceable federal law, depending on whether they are self-executing without additional legislation. The Supreme Court's decision depends upon the dramatic expansion of a narrow but necessary exception to the Supremacy Clause provided in an 1829 Supreme Court precedent. The consequence of that expansion is to put the US historical approach to treaty-making in question. This article provides (a) a brief overview of treaty law in the United States, including the law before Medellín regarding the domestic effect of treaty law, (b) an overview of Medellín, (c) a critique of the Court's reasoning in Medellín and (d) a discussion of its consequences.