Montesquieu
 

XXVI.21 That one must not decide by political laws things that belong to the law of nations

Political laws require that every man be sub­ject to the cri­mi­nal and civil tri­bu­nals of the coun­try where he is, and to cor­rec­tion by the sove­reign.

The law of nations has willed it that prin­ces send ambas­sa­dors to each other, and rea­son drawn from the nature of the thing has not allo­wed these ambas­sa­dors to be sub­ject to the sove­reign of the coun­try to which they are sent, nor to his tri­bu­nals. They are the word of the prince who is sen­ding them, and that word must be free ; no obs­ta­cle must pre­vent them from acting ; they can often annoy because they speak for an inde­pen­dent man ; they could be fra­med for cri­mes if they could be puni­shed for cri­mes ; debts could be impu­ted to them if they could be arres­ted for debts : a prince who has a natu­ral pride would speak through the mouth of a man who would have eve­ry­thing to fear. We must the­re­fore fol­low, with res­pect to ambas­sa­dors, the rea­sons drawn from the law of nations, and not those that derive from poli­ti­cal law. Should they abuse their repre­sen­ta­tive cha­rac­ter, a stop is put to it by sen­ding them home ; they can even be accu­sed before their mas­ter, who the­reby beco­mes their judge or their accom­plice.