On April 21, 2022, the Supreme Court issued a little-noticed but important clarifying decision resolving a lopsided circuit split that held that the substantive choice of law analysis in a case involving the Foreign Sovereign Immunities Act (FSIA) is a function of the forum state's choice of law rule, and not federal common law.

The case, Cassirer v. Thyssen-Bornemisza Collection Found., 142 S. Ct. 1502, 1506 (2022), involved a suit by the descendant of the owner of a stolen Pissarro painting taken by the Nazis and ultimately finding its way into the Thyssen-Bornemisza Collection Foundation, a museum in Madrid, Spain. The plaintiff sued in California under FSIA which permits suits against foreign national entities and sub-units under particular circumstances, notwithstanding sovereign immunity, with exclusive jurisdiction in federal court. The issue was whose property law applied—that of Spain or California—in determining ownership. The trial court and the Ninth Circuit applied federal common law that led the court to apply Spanish law, rather than an analysis under the forum's (California's) choice of law rules that would have led to California law applying.