Montesquieu
 

XXVI.20 That one must not decide by the principles of the civil laws things that belong to the law of nations

Liberty consists prin­ci­pally in never being for­ced to do a thing the law does not com­mand ; and this situa­tion can obtain only because one is gover­ned by civil laws : we are the­re­fore free because we live under civil laws.

If fol­lows from this that prin­ces who do not live among them­sel­ves under civil laws are not free, they are gover­ned by force ; they can conti­nually force or be for­ced. Whence it fol­lows that trea­ties they have made by force are as obli­ga­tory as those they might have made willin­gly. When we who live under civil laws are com­pel­led to make some contract which the law does not require, we can under pro­tec­tion of the law pro­test the vio­lence ; but a prince, who is always in this situa­tion in which he for­ces or is for­ced, can­not com­plain about a treaty that has been impo­sed on him by vio­lence. It is as if he were com­plai­ning about his natu­ral state ; it is as if he wan­ted to be a prince with res­pect to other prin­ces, and the other prin­ces were citi­zens with res­pect to him : in other words, it is to belie the nature of things.