Speaking of the Laws of War, one of the earlier codifications of them was General Orders No. 100, April 1863, "Instructions for the Government of Armies of the United States in the Field."

Art. 28.
Retaliation will, therefore, never be resorted to as a measure of mere revenge, but only as a means of protective retribution, and moreover, cautiously and unavoidably; that is to say, retaliation shall only be resorted to after careful inquiry into the real occurrence, and the character of the misdeeds that may demand retribution.

Unjust or inconsiderate retaliation removes the belligerents farther and farther from the mitigating rules of regular war, and by rapid steps leads them nearer to the internecine wars of savages.
The author of the orders was Francis Lieber, LL.D., a German refugee from the Revolutions of 1848. Prior to the outbreak of the Civil War his antislavery views caused him to have to move away from Charleston, South Carolina. It is possible that Lieber's brief experience as a revolutionary made him more tolerant of irregulars than more traditional interpretations of the "Usages of War."

To read General Orders No. 100 in their entirety, click below:

http://avalon.law.yale.edu/19th_century/lieber.asp