Can the Supreme Court "Develop and Apply Customary International Law"?

Justice Gorsuch raises the question that the majority elides.

Today, the Supreme Court decided Turkiye Halk Bankasi A.S. v. United States. In this case, the United States brought criminal charges against a bank that was an instrumentality of the Turkish government. The bank argued that they had immunity under the Foreign Sovereign Immunities Act. By a 7-2 vote, the Court held that the FSIA only extends to civil cases, and not criminal cases. I don't have strong thoughts on this statutory issue. But the case does not end here.

Justice Kavanaugh's majority opinion left open the possibility that the bank may still be protected by "common-law immunity principles." The Court remanded the case to allow the Second Circuit to "fully consider the various arguments regarding common-law immunity that the parties press in this Court." What exactly is the basis for a common law immunity with regard to international law?

Justice Gorsuch's dissent, which was joined by Justice Alito, identifies one possible answer: customary international law. Gorsuch, however, is not optimistic that the Court could discern a clear answer in customary international law.

The second option—applying customary international law—comes with its own puzzles. If the briefing before us proves anything, it is that customary international law supplies no easy answer to the question whether a foreign sovereign enjoys immunity from criminal prosecution. Compare Brief for Professor Roger O'Keefe as Amicus Curiae 11–16 with Brief for Mark B. Feldman et al. as Amici Curiae 12–13.

I may be biased here. I took international law with Professor Jeremy Rabkin at George Mason. On the first day of class, we read the Declaration of Independence. Professor Rabkin explained that the Declaration was real international law–or, in the lingo, the law of nations. Then, he said (I'm paraphrasing from 15 years ago) that customary international law was whatever the United Nations said. In other words, there is no such thing as customary international law.

In any event, Justice Gorsuch raises a far more foundational question: can federal courts develop customary international law? Gorsuch writes:

Nor is it even altogether clear on what authority federal courts might develop and apply customary international law. Article VI of the Constitution does not list customary international law as federal law when it enumerates sources of "the supreme Law of the Land." And Article I vests Congress rather than the Judiciary with the power to "define and punish . . . Offences against the Law of Nations." §8, cl. 10. See Sosa v. Alvarez-Machain, 542 U. S. 692, 739–742 (2004) (Scalia, J., concurring in part and concurring in judgment); Jesner v. Arab Bank, PLC, 584 U. S. ___, ___–___ (2018) (GORSUCH, J., concurring in part and concurring in judgment) (slip op., at 4–5); Nestlé USA, Inc. v. Doe, 593 U. S. ___, ___ (2021) (GORSUCH, J., concurring) (slip op., at 3).

The modern Supreme Court has held that federal courts are not supposed to develop federal common law. But what about customary international law?

Perhaps Article III incorporated customary international law into federal common law. But since Erie R. Co. v. Tompkins, 304 U. S. 64 (1938), federal courts have largely disclaimed the power to develop federal common law outside of a few reserved areas. See Sosa, 542 U. S., at 740– 742 (opinion of Scalia, J.). And whether customary international law survives as a form of federal common law after Erie is a matter of considerable debate among scholars.Compare C. Bradley & J. Goldsmith, Customary International Law as Federal Common Law: A Critique of the Modern Position, 110 Harv. L. Rev. 815 (1997), with H. Koh, IsInternational Law Really State Law?, 111 Harv. L. Rev. 1824 (1998).

I haven't given this question much thought before. I should now.