Some have proposed the development of technologies that improve our moral behaviour – moral enhancement – in order to address global risks such as pandemics, global warming and nuclear war. I will argue that this technology could be weaponised to manipulate the moral dispositions of enemy combatants. Despite being morally controversial, weaponised moral enhancement would be neither clearly prohibited nor easily prohibitable by international war law. Unlike previous psychochemical weapons, it would be relatively physically harmless. I argue that when combatants are liable to lethal aggression to achieve an aim of war, they are also liable to weaponised moral enhancement to achieve that same aim. Weaponised moral enhancement will loosen just war requirements in both traditional and revisionist normative just war theories. It will particularly affect revisionist theories’ jus ad bellum requirements for humanitarian and preventive wars. For instance, weaponised moral enhancement could be more proportional and efficacious than lethal aggression to effect institutional changes in preventive and humanitarian wars. I will conclude that, despite evading international war laws and loosening normative just war requirements, the intuition that weaponised moral enhancement would gravely harm combatants can be defended by arguing that it would severely disturb personal identity, which could potentially ground future prohibitions.