Relationship of international law and municipal law — Act of State and justiciability — Nature of act of State doctrine in United States law — Contemporary purpose of doctrine — Respect of sovereign independence of State — Judicial restraint in matters of foreign relations — Requirement to establish that case turns on effect of an official action by a foreign sovereign — Burden of proof — Allegation that corporation secured valuable privileges by bribery — Enquiry into motives of foreign government in granting privileges — Whether barred by act of State doctrine — The law of the United States