Under construction.
The sovereignty of the order of Malta is a controversial topic in international law. Many prominent treatises reject it, or take a neutral position. It is hard to find any textbook or treatise that affirms it. In the specialized literature, there are a number of articles supporting the order's sovereignty in international law, others supporting its existence as a subject of international law without necessarily being sovereign, and still others that reject its status as subject of international law.I intend to visit this issue, as a side note to the question of legitimacy of orders. When one of the most legitimate order asserts a doubtful sovereignty, it only makes it harder to distinguish legitimate and illegitimate orders.
In the meantime, see Guy Sainty's position.
International law is a particular area of law. Medieval jurists called it "jus gentium", which is how Romans named the law governing relations between Roman citizens and others (as opposed to "jus civile" which governed relations among citizens). International law governs legal relations (rights and duties) across borders, as opposed to municipal law. Private international law concerns relations between private individuals, while public international law concerns relations that governments have with individuals and with other governments (such as treaties, laws of war, maritime law, human rights, etc).
Many links can be found in the Google directory.
International law is a difficult subject because of its history, its many sources, and the absence of an ultimate arbiter or enforcer. International law arose from medieval "jus gentium", but has evolved considerably over time: thus, the history of a given entity, such as the Order of Malta, can span many centuries and many changes in international law. The sources of international law are treaties, customs, and opinions expressed by judges and jurists: it is difficult to maintain coherence between such diverse sources. Finally, although there are international courts (such as the International Court of Justice founded in 1945, the European Court of Human Rights founded in 1950, etc), there is no court that has a final word and can impose its decisions and make them binding on all.
The aspect of public international law that is relevant here is the question: Who is a subject of international law? A subject of law is an entity (a physical person, a group of persons, a corporation or organization) that has rights and obligations under international law (as opposed to municipal law). A subject of international law can, in principle, enforce its rights or pursue its claims in international courts; it can bind itself with respect to other subjects by entering into agreements; other subjects can hold it to its obligations.
The answer to the question used to be fairly simple: sovereign states were the only subjects of international law. A sovereign state was defined as a specific territory, with a stable population and an organized form of government, that was free to rule itself, and had no constraints or obligations other than those it accepted willingly. This 19th century view of things has long been confronted with exceptions, special cases, grey areas. After 1945, international law has evolved further and accepted that there are other subjects of international law, besides states.
To be completed.An apologist for the order's sovereignty, Berthold Waldstein-Wartenberg, writes (Rechtsgeschichte des Malteserordens, Wien: 1969, Verlag Herold; p. 264) that the sovereignty of the order and its personality in international law is "generally recognized by international law doctrine".
This is a rather excessive statement. The following anthology of opinions on the subject is taken mostly from general textbooks on international law, and show that, at the very least, there is a broad range of opinions.
The French Republic does not recognize the SMOM as a subject of international law; see a statement by the spokesman of the French Ministry of Foreign Affairs, Feb 7, 1997.