On a sense of honour forbidding what the law permits

I must retrace my steps, and must deprive those who wage war of nearly all the privileges which I seemed to grant, yet did not grant to them. For when I first set out to explain this part of the law of nations, I bore witness that many things are said to be ‘lawful’ or ‘permissible’ for the reason that they are done with impunity.

Effect of injustice of a war

[I]f the cause of a war should be unjust, even if the war should have been undertaken in a lawful way, all acts which arise therefrom are unjust from the point of view of moral injustice (interna iniustitia). In consequence, the persons who knowingly perform such acts, or co-operate in them, are to be considered of the number of those who cannot reach the Kingdom of Heaven without repentance. True repentance, again, if time and means are adequate, absolutely requires that he who inflicted the wrong, whether by killing, by destroying property, or by taking booty, should make good the wrong done.

Who are bound to make restitution for an unjust war

Furthermore, according to the principles which in general terms we have elsewhere set forth, those persons are bound to make restitution who have brought about the war, either by the exercise of their power, or through their advice. Their accountability concerns all those things, of course, which ordinarily follow in the train of war; and even unusual things, if they have ordered or advised any such thing, or have failed to prevent it when they might have done so.