States as international persons — In general — Recognition of acts of foreign States and governments — Act of State doctrine — Expropriation of Cuban-owned banks by Republic of Cuba — Effect on deposits Held by banks in United States — Whether Banco Nacional de Cuba entitled to claim as successor to private banks — Whether contrary to United States policy — Circumstances in which United States courts will recognize extraterritorial effect of expropriation decrees — Whether Banco Nacional de Cuba asserting claim on behalf of Government of Cuba — Counterclaim in respect of other acts of Cuban Government — Justiciability — The law of the United States