Again, there are certain duties that we owe even to those who have wronged us. For there is a limit to retribution and to punishment; or rather, I am inclined to think, it is sufficient that the aggressor should be brought to repent of his wrong doing, in order that he may not repeat the offence and that others may be deterred from doing wrong.Then, too, in the case of a state in its external relations, the rights of war must be strictly observed. For since there are two ways of settling a dispute first, by discussion; second, by physical force; and since the former is characteristic of man, the latter of the brute, we must resort to force only in case we may not avail ourselves of discussion.
The only excuse, therefore, for going to war is that we may live in peace unharmed; and when the victory is won, we should spare those who have not been blood-thirsty and barbarous in their warfare. For instance, our forefathers actually admitted to full rights of citizenship the Tusculans, Aequians, Volscians, Sabines, and Hernicians, but they razed Carthage and Numantia to the ground. I wish they had not destroyed Corinth; but I believe they had some special reason for what they did – its convenient situation, probably - and feared that its very location might some day furnish a temptation to renew the war. In my opinion, at least, we should always strive to secure a peace that shall not admit of guile. And if my advice had been heeded on this point, we should still have at least some sort of constitutional government, if not the best in the world, whereas, as it is, we have none at all.
Not only must we show consideration for those whom we have conquered by force of arms but we must also ensure protection to those who lay down their arms and throw themselves upon the mercy of our generals, even though the battering-ram has hammered at their walls. And among our countrymen justice has been observed so conscientiously in this direction, that those who have given promise of protection to states or nations subdued in war become, after the custom of our forefathers, the patrons of those states. As for war, humane laws touching it are drawn up in fetial code of the Roman People under all the guarantees of religion; and from this it may be gathered that no war is just, unless it is entered upon after an official demand for satisfaction has been submitted or warning has been given and a formal declaration made. [...] So extremely scrupulous was the observance of the laws in regard to the conduct of war.
There is extant, too, a letter of the elder Marcus Cato to his son Marcus, in which he writes that he has heard that the youth has been discharged by the consul, when he was serving in Macedonia in the war with Perseus. He warns him, therefore, to be careful not go into battle; for, he says the man who is not legally a soldier has no right to be fighting the foe. This also I observe – that he who would properly have been called «a fighting enemy» (perduellis) was called «a guest» (hostis), thus relieving the ugliness of the fact by a softened expression; for «enemy» (hostis) meant to our ancestors what we now call «stranger» (peregrinus). This is proved by the usage in the Twelve Tables: «Or a day fixed for trial with a stranger» (hostis). And again: «Right of ownership is inalienable for ever in dealings with a stranger» (hostis). What can exceed such charity, when he with whom one is at war is called by so gentle a name? And yet long lapse of time has given that word a harsher meaning: for it has lost its signification of «stranger» and has taken on the technical connotation of «an enemy under arms».