the Law of Nations first.
To summarize, the author, Vattel recognizes that there are several forms of government but he states that the principles contained apply to all.
ยง 1. Of the state, and of sovereignty
A NATION or a state is, as has been said at the beginning of this work, a body politic, or a society of men united together for the purpose of promoting their mutual safety and advantage by their combined strength.
From the very design that induces a number of men to form a society which has its common interests, and which is to act in concert, it is necessary that there should be established a Public Authority, to order and direct what is to be done by each in relation to the end of the association. This political authority is the Sovereignty; and he or they who are invested with it are the Sovereign.
http://www.constitution.org/cmt/law_of_nations.htm