State immunity — Jurisdictional immunity — Nature of immunity — Absence of immunity a jurisdictional requirement — Exception to immunity — Non-sovereign acts — Assassination of United States national working in Iran — Whether falling within exception for non-commercial torts — Whether falling within commercial activity exception in view of deceased's employment — Requirement of direct effect within the United States — Requirement of substantial connection between murder and commercial activity — Waiver — Iran-United States Treaty of Amity, 1955 — The law of the United States