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 for sovereign independence of States — Judicial restraint in matters of foreign relations — Allegations that United States corporation secured oil concessions by bribery — Inquiry into motives of foreign government in granting concessions — Whether barred by act of State doctrine — Exceptions to doctrine — Commercial activity exception — Whether part of United States law — Whether act of granting concession a commercial activity — United States Foreign Corrupt Practices Act — Public prosecution — Whether contrary to act of State doctrine — Difference between public and private actions — The law of the United States