One of our oldest laws, the Alien Tort Statute (1789), grants federal courts jurisdiction over lawsuits brought by aliens for actions “in violation of the law of nations.” Courts have differed in their method of interpreting this “law of nations” — an old way of saying “international law” — and thus in their decisions on what behavior violates it and the types of defendants who may be liable. Recent ATS litigation has thus ignited a debate over the role of judges in applying international law. Kiobel v. Royal Dutch Petroleum presents the question of whether, under the ATS, the law of nations can be applied against an entity that is not a natural person: a corporation. In this case, 12 Nigerians sued Royal Dutch and its Shell subsidiaries, alleging that Nigerian soldiers committed human rights abuses on the companies’ behalf between 1992 and 1995, purportedly in response to demonstrations against oil exploration. The district court dismissed most of the claims but let certain others proceed. The Second Circuit dismissed the case entirely, holding that the ATS’s jurisdictional grant does not extend to cases against corporations, which are not liable for crimes under the law of nations. The Supreme Court agreed to review the case. Cato has now filed a brief arguing that the ATS must be interpreted in a manner consistent with Congress’s original jurisdictional grant. This interpretation, supporting the Second Circuit’s ruling, maintains the Constitution’s separation of powers — which gives Congress the power to determine the scope of federal courts’ jurisdiction. Allowing courts to expand their jurisdiction without Congress’s consent would create a “democracy gap” that would be particularly serious here, where the case involves issues of foreign affairs that are appropriately the province of the political branches. The Supreme Court made clear in Grupo Mexicano de Desarrollo, S.A. v. Alliance Bond Fund, Inc. (1999) that evolving methods of interpreting international law do not inform the ATS’s jurisdictional reach, which has not changed since 1789. Nonetheless, lower courts are split on whether corporations may be liable for the sorts of violations at issue here, largely due to their varied interpretive methods. In our brief, we urge the Court to clarify the proper method of interpreting the law of nations under the ATS. We argue that Judge José Cabranes, a leading international law jurist (and Justice Sonia Sotomayor’s mentor) who authored the Second Circuit’s Kiobel decision, set out the correct interpretive method in an earlier case, Flores v. Southern Peru Copper Corp. (2003). Judge Cabranes’s reasoning in Flores embodied both the guidance that the Supreme Court would give in Sosa v. Alvarez‐Machain (2004) and the teachings of classical theorists like Grotius, by defining customary international law as “composed only of those rules that States [countries] universally abide by, or accede to, out of a sense of legal obligation and mutual concern.” Judge Cabranes used as relevant evidence the States’ formal lawmaking actions, such as international conventions that “establish rules expressly recognized by the contesting states” and international custom where the States adhere “out of a sense of legal obligation.” He further acknowledged that the method used in 1789 to interpret what comprised the law of nations defined both the claims and the parties cognizable under international law. By looking to the proper sources, Judge Cabranes correctly concluded that corporations cannot be held liable for violations of international law for ATS purposes, and in so doing recognized the constitutional checks that prevent courts from expanding their own jurisdiction.