- Mar 7, 2009
Prior notification was something that appears to have been an added requirement by Leiber's code which as you've noted was not in existence when the retaliatory act occurred November 1862. Vattel's Law of Nations that included the laws of war, had no requirement for prior notification that retaliation was going to take place. Vattel indicates that a notification be given that further acts of retaliation may continue if the enemy continues to disregard the laws of war:Agreed to cite the Lieber Code (which to be fair was not yet in effect but its sentiments in the atmosphere)
Art. 27: The law of war can no more wholly dispense with retaliation than can the law of nations, of which it is a branch. Yet civilized nations acknowledge retaliation as the sternest feature of war. A reckless enemy often leaves to his opponent no other means of securing himself against the repetition of barbarous outrage
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.
That being said, if this did happen and Union authorities did not declare it to Forrest's command or the rebel government in a way similar to Mosby's letter to Sheridan one could very easily make the argument that it was not done cautiously as a means of protective retribution and rather as revenge and thus "war crime". The code may not stipulate declaring an act of retaliation but if you don't inform the enemy I don't see how you can argue you did it for the purposes of motivating them to cease and desist future injustices. If they did declare it, finding such a letter may make a fun research project for someone with the time and interest.
"This leads us to speak of a kind of retaliation sometimes practised in war, under the name of reprisals. If the hostile general has, without any just reason, caused some prisoners to be hanged, we hang an equal number of his people, and of the same rank,—notifying to him that we will continue thus to retaliate, for the purpose of obliging him to observe the laws of war. It is a dreadful extremity thus to condemn a prisoner to atone, by a miserable death, for his general’s crime: and if we had previously  promised to spare the life of that prisoner, we cannot, without injustice, make him the subject of our reprisals. Nevertheless, as a prince or his general has a right to sacrifice his enemies’ lives to his own safety and that of his men, —it appears, that, if he has to do with an inhuman enemy who frequently commits such enormities, he is authorised to refuse quarter to some of the prisoners he takes, and to treat them as his people have been treated."
Emerich Vattel's Law of Nations, Book III, Chapter VIII, Section 142