Hugo Grotius states that in an unjust war any acts of hostility done in that war are “unjust in themselves” (1625)
Found in: The Rights of War and Peace (2005 ed.) vol. 3 (Book III)
Hugo Grotius attempted to codify the historical, moral, and legal grounds for justly waging war against an enemy. Here are his thoughts on acts committed in an unjust war:
War & Peace
III. We then first declare, if the Cause of the War be unjust, tho’ it be undertaken in a solemn Manner, yet all the Acts of Hostility done in it are unjust in themselves. So that they who knowingly do these Acts, or join in the acting of them, Are to be accounted in the Number of those, who without Repentance cannot enter into the Kingdom of Heaven, 1 Cor. vi. 10. But true Repentance, if Opportunity and Ability will allow, absolutely requires that he who has done any Damage, either by killing, ravaging or plundering, should make full Restitution.