The Supremacy Clause and International Law: A Response to Jordan Paust Commentary
The Supremacy Clause and International Law: A Response to Jordan Paust
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JURIST Guest Columnist David Moore of the Brigham Young University J. Reuben Clark Law School says that although the Supremacy Clause binds states to follow international law that qualifies as preemptive federal law, not all treaties or principles of customary international law qualify as such…

Professor Jordan Paust asserts that states cannot pass laws that prohibit their courts from applying either of the two primary sources of international law — treaties (agreements negotiated and ratified by states) or customary international law (principles that states generally and consistently follow because they feel legally bound to do so even though the principles may not be codified in a treaty). Professor Paust is correct, but only to a point. The Supremacy Clause of the US Constitution binds states to follow international law that qualifies as enforceable, preemptive federal law. But not all treaties or principles of customary international law qualify.


Early in our history, the US Supreme Court in Foster v. Neilson recognized that only certain treaties — treaties that are self-executing — are immediately enforceable in US courts. Other treaties — non-self-executing ones — must be executed or implemented by a federal statute before they can be judicially enforced to preempt state law. In the Court’s words:

Our constitution declares a treaty to be the law of the land. It is, consequently, to be regarded in courts of justice as equivalent to an act of the legislature, whenever it operates of itself without the aid of any legislative provision. But when the terms of the stipulation import a contract, when either of the parties engages to perform a particular act, the treaty addresses itself to the political, not the judicial department; and the legislature must execute the contract before it can become a rule for the Court.

The result is that states may enact laws inconsistent with non-self-executing treaties until those treaties are executed. The Court’s recent decision in Medellín v. Texas confirms and illustrates this point. Mexican national Jose Ernesto Medellín was sentenced to death in Texas for his participation in the gang rape and murder of two teenage girls. The US, however, failed to notify Medellín of his right to contact the Mexican consulate for help in his criminal prosecution as required by the Vienna Convention on Consular Relations (Vienna Convention). In response to a suit brought by Mexico, the International Court of Justice (ICJ) held that the US had violated its treaty commitments under the Vienna Convention and was obligated to review and reconsider the convictions and sentences of Medellín and other Mexican nationals. The Court acknowledged that the ICJ judgment was binding on the US as a matter of international law. However, the Court concluded that the judgment did not preempt Texas procedural default law, under which Medellín’s Vienna Convention claim was barred, because the US treaty obligation to comply with ICJ judgments was non-self-executing.

The Supreme Court in Foster and Medellín saw no conflict between the notion that some treaties are non-self-executing and the Supremacy Clause, which declares treaties to be the supreme law of the land. The two are easily reconciled. The Supremacy Clause was adopted to ensure that federal authorities could adopt laws and treaties to which the states would be bound. The Clause does not require that all laws, treaties or even constitutional provisions preempt state law. For example, the Bill of Rights, notwithstanding its stature as part of the US Constitution, only binds the federal government. It took the Civil War amendments to the constitution to render some of the Bill of Rights provisions applicable to the states. Similarly, Congress enacts statutes that expressly or implicitly refuse to preempt state law, even though statutes also qualify as supreme federal law. The upshot is that federal law and treaty-makers may adopt statutes and ratify treaties that do less than the Supremacy Clause would allow. States may pass laws inconsistent with such statutes and treaties until federal authorities move to make them preemptive. For further reference, please see my article Law(makers) of the Land: The Doctrine of Treaty Non-Self-Execution [PDF].

Customary International Law

Just as treaties do not uniformly qualify as judicially enforceable federal law, it is far from clear that customary international law is preemptive, federal law. The status of customary international law under US law has been hotly debated and I previously commented on this issue in An Emerging Uniformity for International Law. As noted above, customary international law emerges when states engage in a general and consistent practice that they come to feel is legally binding upon them. Thus, customary international law is not made through majority vote of both houses of Congress and presidential approval (or through a supermajority vote of both houses following a presidential veto) like other “laws of the US” that are rendered supreme by way of the Supremacy Clause. Professor Paust is right that federal courts have applied customary international law in the past and that the Supreme Court has repeatedly said that the law of nations is part of US law. However, as I have previously explained [PDF], customary international law was historically considered part of the general law that federal and state courts alike applied without binding the other. Rejecting the notion that there exists a body of general law unattached to any particular government authority, the Supreme Court in Erie v. Tompkins jettisoned general federal law. The result, arguably, is that customary international law is preemptive federal law only when the Constitution or federal statutes so indicate.

The Court appears to have endorsed this result in Sosa v. Alvarez-Machain. Mexican national Humberto Alvarez-Machain was believed to have participated in the torture and murder of a US agent in Mexico. After unsuccessful negotiations with the Mexican government, the Drug Enforcement Administration (DEA) authorized a plan under which fellow Mexican national Jose Francisco Sosa kidnapped Alvarez-Machain in Mexico and brought him to the US to face charges. Those charges were dismissed and Alvarez-Machain sued Sosa in federal court under a statute called the Alien Tort Statute (ATS) for violating a customary international law prohibition on arbitrary detention. The Court reasoned that the ATS provides federal courts jurisdiction to hear claims for violation of customary international law. The Court also recognized that “[f]or two centuries, we have affirmed that the domestic law of the United States recognizes the law of nations.” However, the Court did not conclude that all customary international law is federal law that preempts contrary state law. Instead, the Court concluded, based on congressional intent in enacting the ATS and the judicial changes wrought by Erie Railroad Co. v. Tompkins, that federal courts have limited authority to recognize causes of action based on customary international law. Federal courts may only recognize causes of action based on norms of customary international law that are as specifically defined and well accepted as the norms Congress had in mind when enacting the ATS. The Court’s reasoning in reaching this conclusion suggested that customary international law norms are not federal law absent incorporation into domestic law. If this reading of Sosa is correct, customary international law does not qualify as preemptive, federal law in the absence of constitutional or statutory authority to the contrary. Consequently, customary international law alone does not prevent the states for legislating in ways inconsistent with custom. For example, the customary international law prohibition on the death penalty, even if applicable to the US, does not prevent states from authorizing capital punishment.

In short, while Professor Paust is correct to say that federal law can preempt state law, treaties and customary international law do not necessarily and immediately qualify as enforceable federal law.

David Moore is a Professor of Law at Brigham Young University Law School. He has taught courses on international law, US foreign relations law and international human rights law. Professor Moore clerked for Justice Samuel Alito during the US Supreme Court’s 2007 term. His articles have appeared in the Harvard Law Review, Columbia Law Review, Northwestern University Law Review, and the UCLA Law Review among others.

Suggested citation: David Moore, The Supremacy Clause and International Law: A Response to Jordan Paust, JURIST – Forum, July 1, 2012,

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