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As I discuss in my forthcoming book, International Law in the U.S. Legal System, regardless of whether customary international law has the status of self-executing federal law, it can play an important role in U.S. litigation. The invocation of customary international law in civil suits brought under the Alien Tort Statute is of course well known and the matter of significant debate, and we will soon learn more about the future scope of this litigation once the Supreme Court decides Kiobel. Increasingly, however, customary international law is also playing an important role in the criminal area. Three recent decisions, involving alleged terrorists, pirates, and drug traffickers, illustrate this phenomenon. Underlying these decisions is an unresolved question of deference that is likely to become a more prominent issue going forward. Readers of Lawfare are of course familiar with the D.C. Circuit’s decision from October in Hamdan v. United States, in which the court concluded that material support for terrorism could not be prosecuted under the Military Commissions Act of 2006. The court reasoned that, because of ex post facto concerns, Congress had not wanted to criminalize conduct pre-dating the Military Commissions Act unless that conduct already constituted an offense under the customary international laws of war, and it found that material support for terrorism did not constitute such an offense. Despite observing that “[i]t is often difficult to determine what constitutes customary international law, who defines customary international law, and how firmly established a norm has to be to qualify as a customary international law norm,” the court expressed the view that “[t]he content of customary international law is quite evident” on the issue. Hamdan involved a statutory scheme that was found by the court to be centered around the law of nations. But customary international law can also play a role in limiting criminal prosecutions through the more general Charming Betsy canon of construction, pursuant to which courts will attempt to construe statutes to avoid violations of international law. Consider, for example, the decision by the federal district court in D.C. last July in United States v. Ali. The issue there was whether a Somali citizen could be prosecuted after acting as an intermediary between Somali pirates and the Dutch owner of a Bahamian vessel that had been apprehended by the pirates in international waters. Applying the Charming Betsy canon, the court concluded (among other things) that the defendant could not be prosecuted either for acts of aiding and abetting piracy that did not occur on the high seas or for an alleged conspiracy to commit piracy. The court found that, unlike acts of piracy, these other offenses did not constitute universal jurisdiction offenses under customary international law and thus could legitimately be punished only if the prosecuting state had either a territorial connection to the conduct or a nationality connection to the perpetrators or victims, and it found no indication that Congress had intended to override that international law requirement. This case is currently on appeal to the D.C. Circuit. In addition to affecting the interpretation of federal statutes, the content of customary international law can affect Congress’s constitutional authority to define and punish offenses against the law of nations. This was evident in a decision in November by the Eleventh Circuit in United States v. Bellaizac-Hurtado. In that case, the court held that Congress lacked the power, under the Maritime Drug Law Enforcement Act, to authorize a drug trafficking prosecution for non-U.S. citizens apprehended in the territorial waters of Panama. The only constitutional basis invoked by the government for applying the Act in this situation was Congress’s authority to define and punish offenses against the law of nations, and the court concluded that Congress was limited under this authority to punishing conduct recognized as an offense under customary international law. The court reasoned that it did not need to determine whether the conduct had to be an offense under customary international law at the time the Constitution was adopted, because it concluded that even today drug trafficking is not an offense under customary international law. (In a concurrence, Judge Barkett also reasoned that Congress’s power to define and punish offenses against the law of nations was limited by customary international law requirements of prescriptive jurisdiction, which she argued were not met in this case.) In each of these decisions, the courts examined a variety of materials in an effort to discern the customary practices and understandings of the international community, and the courts applied their interpretation in a way that limited the bounds of U.S. criminal prosecutions. One question implicated by this sort of judicial reliance on customary international law—although not a question that was specifically addressed in these cases—is how much deference the courts should give to the judgment of the Executive Branch about the content of customary international law. The question was not at issue in Hamdan, since the Executive Branch did not argue on appeal that material support for terrorism violated the international laws of war, but it was potentially relevant to the other two cases. The court in Bellaizac-Hurtado denied (correctly, in my view) that the political branches have unlimited authority to label something a violation of customary international law, but that should not necessarily rule out deference to reasonable political branch interpretations. The Supreme Court has stated on a number of occasions that it will give “great weight” to Executive Branch interpretations of treaties (although it admittedly seemed not to apply such deference in its own Hamdan decision in 2006 when interpreting Common Article 3 of the Geneva Conventions). The issue is somewhat more complicated for customary international law, since part of the reason for deference on treaty questions is that the Executive Branch plays a direct role in negotiating U.S. treaties, whereas the Executive Branch’s role in the development of customary international law is more diffuse. In criminal cases, there might also be a separation of powers concern associated with allowing the Executive Branch both to act as the prosecutor and giving it deference on the content of a body of law that affects its prosecutorial authority (although that concern would also apply in some cases involving criminal statutes that implement treaties). On the other hand, for both treaties and customary international law it may be desirable as a general matter for the United States to be able to speak with a unified voice, since whatever position the U.S. adopts may have reciprocity and other international consequences. (For this reason, the Restatement (Third) of Foreign Relations Law suggests that there should be judicial deference for both treaty and customary international law questions.) It is also generally more difficult for courts to discern the content of customary international law than the content of treaties, which might justify giving greater deference to the Executive Branch with respect to custom, not less. In any event, as courts continue to consider cases in which customary international law is a potential constraint on criminal prosecution, the issue of deference is likely to emerge as a more significant issue.
Curtis Bradley is the Allen M. Singer Professor at the University of Chicago Law School. His courses include Foreign Relations Law and Federal Courts. He joined the Chicago faculty in 2021, after having taught for many years at Duke Law School. He has served as Counselor on International Law in the Legal Adviser’s Office of the U.S. State Department and as a Reporter for the Restatement of Foreign Relations.
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