Relationship of international law and municipal law — Act of State and justiciability — United States act of State doctrine — Nature of act of State doctrine in United States law — Requirement of sovereign independence of State — Judicial restraint in matters of foreign relations — Antitrust investigations — Companies required to divulge communications with foreign government — Possibility of enquiry into the motives behind the granting of shipping rights by foreign governments in later proceedings — Whether requirement to divulge communications barred by act of State doctrine — The law of the United States