Srimad Bhagavatam Canto 01, Chapter 07, Text 36

SB 1.7.36

mattam pramattam unmattam
suptam balam striyam jadam
prapannam viratham bhitam
na ripum hanti dharma-vit
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
A person who knows the principles of religion does not kill an enemy who is careless, intoxicated, insane, asleep, afraid or devoid of his chariot. Nor does he kill a boy, a woman, a foolish creature or a surrendered soul.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
An enemy who does not resist is never killed by a warrior who knows the principles of religion. Formerly battles were fought on the principles of religion and not for the sake of sense gratification. If the enemy happened to be intoxicated, asleep, etc., as above mentioned, he was never to be killed. These are some of the codes of religious war. Formerly war was never declared by the whims of selfish political leaders; it was carried out on religious principles free from all vices. Violence carried out on religious principles is far superior to so-called nonviolence.
Srimad Bhagavatam Canto 01, Chapter 07, Text 35
Srimad Bhagavatam Canto 01, Chapter 07, Text 37