State immunity — Jurisdictional immunity — Commercial activity exception — Action for breach of contract against foreign State entity — Requirement of connection between commercial activity of foreign State entity and United States — Requirement that cause of action be based on that commercial activity — Commercial activity of foreign entity relied upon by plaintiff relating to original contract between parties — Action based upon later agreement between parties — Whether commercial activity relating to earlier agreement sufficient to establish exception to immunity in suit based on later agreement — The law of the United States