A more thorough explanation:

CONTRA JUS BELLI

Contra jus belli means "against the law of war" in Latin.

Examples of actions that are contra jus belli include targeting civilians, using chemical weapons, and torturing prisoners of war.

Contra jus belli refers to actions that violate the laws and customs of war. These actions are considered illegal and unethical, and can result in war crimes charges. For example, targeting civilians is a violation of the principle of distinction, which requires combatants to distinguish between military targets and civilians. Using chemical weapons is prohibited by the Chemical Weapons Convention, which bans the production, stockpiling, and use of chemical weapons. Torturing prisoners of war is a violation of the Geneva Conventions, which require humane treatment of prisoners of war.