Allow me to make - actually, restating my other posts - that Marten's discussion of "law of nations" and the "law of war" does not create a very rigorous framework.
In part, the premise behind Marten (and Vattel) is that the laws of war apply to authorized combatants and the civilians should stay out of the way (the become "banditti").
Even for such things as propery, see the quote below (from pages 295-296 of Martens 1802 - Vattel is similar, only with longer text). Despite the use of many words, the language comes down to nearly anything goes (I did say nearly). - R
"The conqueror has a right to seize on all the property of the enemy that comes within his power: it matters not whether it be immoveable (conquéte eroberung), or moveable, (butin, beute, booty). These seizures may be made, 1. in order to obtain what he demands as his due, or an equivalent; 2. to defray the expenses of the war; 3. to force the enemy to an equitable peace; 4. to deter him, or by reducing his strength, hinder him, from repeating, in future, the injuries which have been the. cause of the war. And, with this last object in view, a power at war has a right to destroy the property and possessions of the enemy, for the express purpose of doing him mischief.
However, the modern laws of war do not permit the destruction of any thing, except, 1. such things as the enemy cannot he deprived of by any other means than those of destruction, and which it is at the same time necessary to deprive him of; 2. such things as after being taken, cannot be kept, and which might, if not destroyed, strengthen the enemy; 3. such things as cannot be preserved without injury to the military operations. To all these we may add, 4. whatever is destroyed by way of retaliation."