Whether it is permissible for individuals to do harm to a public enemy

What I have heretofore said applies chiefly to those who either possess the supreme command in war or are carrying out public orders. We must also consider what is permissible for an individual in war, not only according to natural and divine law, but also according to the law of nations.

In his first book On Duties, Cicero says that the son of the Censor Cato had served in the army of the general [Popillius], but that the legion in which he was serving was disbanded; nevertheless, since the youth from love of warfare remained in the army, Cato wrote to Popillius that he ought to oblige the young man to take the military oath a second time, if he wished him to remain in the army. Cato gave as a reason that after the first oath had been cancelled his son could not lawfully fight with the enemy. Cicero adds the very words of Cato from a letter to his son, in which he warns the youth to avoid engaging in battle, for the reason that it is not right for one who is not a soldier to fight with an enemy.

But those are deceived who think that the principle thus stated has its origin in the law of nations. This becomes clear if you consider that, just as anyone is permitted to seize the property of an enemy, so also, as we have shown above, it is permissible to kill an enemy. For according to the law of nations, enemies are held to be entitled to no consideration.