State immunity — Jurisdictional immunity — Criteria for establishing whether entity is a foreign State or an agency or instrumentality of a foreign State — Proceedings against air force of a foreign State — Whether service of process effective — Foreign Sovereign Immunities Act 1976 — Different modes of service for foreign States and for agencies and instrumentalities of foreign States — Whether status of entity to be determined by reference to its commercial activities or existence of separate legal personality — Whether a presumption that military entities constitute foreign States for purpose of proceedings — The law of the United States