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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section II : On the armed force of belligerent states - Art. 13.
Art. 13. Population of unoccupied territory. The inhabitants of a territory which has not been occupied who, upon the approach of the enemy, spontaneously arm vessels to fight him, without having had time to convert them into war-ships in conformity with Articles 3
and following, shall be considered as belligerents, if they act openly and if they respect the laws and usages of war.