Relationship of international law and municipal law — Conduct of foreign relations — Act of State — Whether applicable to claim for breach of contract — Whether claim calling into question validity of acts of a foreign government

State immunity — Jurisdictional immunity — Commercial activity exception — Action for damages for breach of contract against a State entity — Whether Government can be held liable for acts of State entity — Agency relationship — Distinction between State entity and alter ego of the Government — Philippines Presidential Commission on Good Government — Contract for sale of aircraft — Failure by State entity to fulfil contractual obligation to defend purchaser of aircraft in event of challenge to validity of title to aircraft — The law of the United States