States as international persons — In general — Recognition of acts of foreign States and governments — Act of State doctrine — Expropriation — Scope of expropriation — Opinion of law officer of foreign State concerning scope — Whether conclusive — Whether opinion an act of State — Commercial activities — Government engaging in commercial activities with expropriated property — Whether depriving expropriation of its commercial character — Inaction by government agency — Whether capable of constituting an act of State — The law of the United States