State responsibility — Nature and kinds of — For revocation of, or interference with, concessions or concessionary contracts — Nature of rights under oil concession — Whether contractual or proprietary — Title to oil prior to extraction — Governing law — Whether nationalization of concession is an expropriation of oil yet to be extracted — Act of State doctrine — Hickenlooper Amendment — The law of the United States

States as international persons — In general — Recognition of acts of foreign States and governments — Act of State doctrine — Nationalization of oil concession — Whether an act of State — Hickenlooper Amendment — Scope and conditions of application — Requirement that plaintiff assert a claim of title or a property right — Rights under oil concession — Whether contractual or proprietary — Governing law — Concession containing choice of law provision referring to more than one legal system — Whether choice of law clause determines nature of concession rights or merely enforcement — Nature of concession — Whether nationalization a violation of concession — Exceptions to the act of State doctrine — The law of the United States