What is it?
Customary international law, also known as public international law, is the body of law determined by the “general and consistent practice of states followed by them from a sense of legal obligation.” Restatement (Third) of the Foreign Relations Law of the United States S102(c)(2). In other words, international law need not arise exclusively from treaties and formal international agreements; it may also arise from an aggregate of states’ actions, states’ official statements, and international legal obligations arising from mutual concerns.
Why do I care?
Whereas treaties and other international agreements are not necessarily self-executing (in that they often require Congressional approval or specific statutes to become binding US law), customary international law is a binding, valid part of United States federal common law. See The Paquete Habana (S.Ct. 1900); Banco Nacional de Cuba v. Sabbatino (S.Ct. 1964). However, this body of law is for the most part completely inaccessible.
Bringing a cause of action based on customary international law is nearly impossible, and prevailing on such an action carries an even slimmer chance of success. Based a cursory review of Federal case law, one could easily come to the conclusion that the only scenarios under which customary international law can successfully be used to uphold individual rights are those that involve fish, kidnapped anglers, and the Spanish-American War. The Paquete Habana (S.Ct. 1900).
So why the lack of application?
There is no question that customary international law is a valid part of federal common law. Even the Rehnquist court, despite its ideological fault-lines, found consensus on the validity of customary international law as a part of federal common law in Sosa v. Alvarez-Machain (2004):
For two centuries we have affirmed that the domestic law of the United States recognizes the law of nations. It would take some explaining to say now that federal courts must avert their gaze entirely from any international norm intended to protect individuals.
Unfortunately one significant problem was – and continues to be – a rather important issue. How do we define what, exactly, customary international law dictates?
Modern Second Circuit vs. Ye Olde SCOTUS
Customary international law was first recognized as a part of federal common law in the landmark case of The Paquete Habana (S.Ct. 1900), and later upheld for the post-Erie world by Banco Nacional de Cuba v. Sabbatino (S.Ct. 1964). In the former case, two Spanish fishing boats were captured by the naval blockade surrounding Cuba during the Spanish-American war. The ships and their cargo were taken back to Florida where they were sold at auction as “spoils of war.” The Court, in ruling that such a seizure was a violation of customary international law relied upon the following treatise by Carlos Testa a captain in the Portuguese navy and a legal scholar of the time:
Nevertheless, in this, customary law establishes an exception of immunity in favor of coast fishing vessels. Fishing is so peaceful an industry, and is generally carried on by so poor and so hardworking a class of men, that it is likened, in the territorial waters of the enemy’s country, to the class of husbandmen who gather the fruits of the earth for their livelihood. The examples and practice generally followed establish this humane and beneficent exception as an international rule, and this rule may be considered as adopted by customary law and by all civilized nations.
However, in United States v. Yousef (2003) the Second Circuit rejected such reliance on the work of scholars for clarification of customary international law, declaring such statements as usurpation of legal power by non-state actors if relied upon as anything more than supplementary evidence that relevant customary international law existed.
This notion—that professors of international law enjoy a special competence to prescribe the nature of customary international law wholly unmoored from legitimating territorial or national responsibilities, the interests and practices of States, or (in countries such as ours) the processes of democratic consent—may not be unique, but it is certainly without merit.
Thus, according to the Second Circuit, clarifications of customary international law are invalid as bases of decision. And when Yousef appealed, the Supreme Court declined to grant certiorari.
Where does this leave customary international law? Can we rely on the work of legal scholars to clarify customary international law as we do for domestic common law under the Restatements? Or is reliance of such clarifications a usurpation of power from national and international actors?
If the former, does this place us at the mercy of academics in deciding matters of international law and diplomacy? If the latter, does a rejection of clarification render this entire body of law unenforceable and functionally useless?
For further reading, see “International Law as Law in the United States”, Michigan Law Review Vol. 82, No. 5/6 (1984); Litigation of International Disputes in U.S. Courts, 2d § 9:2.