Finding Customary International Law

I. IntroductionQuestions about customary international law ("CIL") arise in a wide variety of federal litigation. For example, the Alien Tort Statute ("ATS") provides the district courts with jurisdiction over "any civil action by an alien for a tort only, committed in violation of the law of nations or a treaty of the United States."1 In Sosa v. Alvarez-Machain, the Supreme Court held that the statute's reference to the "law of nations"-the old term for CIL-empowers federal courts to recognize common law causes of action for violations of international norms that are "accepted by the civilized world" and defined with adequate specificity.2 Plaintiffs have since relied on the ATS to bring tort claims against multinational corporations, foreign and U.S. officials, governments, and private individuals for torture, extrajudicial killing, crimes against humanity, and war crimes, among other acts.3 In many of these cases, a central question is whether custom prohibits the alleged conduct.As another example, the Foreign Sovereign Immunities Act ("FSIA") provides that foreign states are immune from the jurisdiction of federal and state courts unless a statutory exception to immunity applies.4 One such exception strips immunity in certain cases "in which rights in property taken in violation of international law are in issue."5 To decide whether this language applies, federal courts must often identify the circumstances in which CIL prohibits governmental takings of private property.6Similar issues arise in criminal cases. one federal statute establishes a penalty of life in prison for anyone who, "on the high seas, commits the crime of piracy as defined by the law of nations, and is afterwards brought into or found in the United States."7 In applying this text, courts have looked to CIL to resolve issues such as whether piracy encompasses acts of aiding and abetting,8 and whether it includes acts of violence that do not accompany a robbery.9 Elsewhere, judicial efforts to discern custom are often necessary to apply the so-called Charming Betsy canon, which requires a court to interpret an ambiguous statute in a way that avoids conflict with international law.10Finally, questions about CIL arise in constitutional interpretation. In United States v. Bellaizac-Hurtado, for example, the government prosecuted several foreign nationals under the Maritime Drug Law Enforcement Act ("MDLEA") for engaging in drug trafficking in the territorial waters of Panama.11 In response, the defendants argued that the statutory prohibition is unconstitutional-while Article I grants Congress the power to "define and punish . . . Offences against the Law of Nations,"12 and while that power confers authority to criminalize conduct that CIL prohibits,13 custom does not prohibit drug trafficking.14 Persuaded by this argument, the court invalidated the statute as applied and vacated the convictions.15As a matter of international law and federal common law, it is clear what courts should do when faced with these kinds of issues. The traditional doctrine is that a rule of CIL arises from a "general and consistent practice of states followed by them from a sense of legal obligation."16 A correct analysis would thus seek to answer two questions: first, is there a general and consistent state practice on the given issue? And second, to the extent such a practice exists, have states engaged in it out of a sense of international legal obligation (otherwise known as opinio juris)? With respect to the first question, the predominant view is that a practice cannot be "general" unless, at a minimum, a "large share of [the] affected states" have engaged in it,17 that the requirement of consistency necessitates steady adherence even if not "absolutely rigorous conformity,"18 and that the occurrence of a practice over a substantial period of time supports, but is not strictly necessary for the development of a norm.19 It is also widely accepted that state practice can take diverse forms, including diplomatic acts, statutes, judicial decisions, official statements, and regulations, among others. …