Hugo Grotius states that in an unjust war any acts of hostility done in that war are “unjust in themselves” (1625)
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:
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.
In this passage Grotius argues that if the cause of a war be unjust, then any acts of hostility done in that war are also unjust; and the party which does any damage in that war must pay full restitution. These thoughts remind one of what Bates says to King Henry on the eve of battle in Shakespeare’s play Henry V.