Declarations of war are intimations less to powers than to their subjects. The foreigner, whether king, individual, or people, who robs, kills or detains the subjects, without declaring war on the prince, is not an enemy, but a brigand.
 Jean-Jacques Rousseau, The Social Contract (1762). copy citation

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Author Jean-Jacques Rousseau
Source The Social Contract
Topic war foreigners
Date 1762
Language English
Reference Of the Social Contract, or Principles of Political Law, Book I
Note Translated by George Douglas Howard Cole
Weblink https://en.wikisource.org/wiki/The_Social_Contract/Book_I

Context

“Finally, each State can have for enemies only other States, and not men; for between things disparate in nature there can be no real relation.
Furthermore, this principle is in conformity with the established rules of all times and the constant practice of all civilised peoples. Declarations of war are intimations less to powers than to their subjects. The foreigner, whether king, individual, or people, who robs, kills or detains the subjects, without declaring war on the prince, is not an enemy, but a brigand. Even in real war, a just prince, while laying hands, in the enemy's country, on all that belongs to the public, respects the lives and goods of individuals: he respects rights on which his own are founded.” source