Relationship of international law and municipal law — Act of State and justiciability — United States act of State doctrine — Applicability — United States courts barred from questioning validity of acts of foreign State within own jurisdiction — Scope of doctrine — Underlying principles — Allegation that United States corporation obtained contract in Nigeria by bribery — Whether act of State doctrine applicable — Comity — Relevant to decision whether to apply doctrine in case where it is applicable — Not ground for extending application of doctrine to new areas — The law of the United States