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.

Much of the history of twentieth century international law was shaped by efforts to reduce recourse to the use of force through the codification of a restrictive doctrine of ‘just war’. After September 11th, 2001, the US Administration under George W. Bush has sought to resurrect an expansive doctrine of just war, one rooted in broad moral, rather than restrictive legal, assessments of threats and punishments. Existing rules ask us to pause and inquire whether war is necessary and just. The debate over Iraq laid bare failings in these rules, requiring action. Yet the need to limit resort to war is as great as ever. Legal rules cannot prevent the use of force; nor can they prevent violations that states perceive to be in their fundamental interests. Rather, international law provides a framework against which states' actions are assessed, and imposes a heavy burden of justification. Is a given war justified as self-defense? Is a state asserting a need to create a new legal category of humanitarian intervention, based upon a responsibility to protect? Does it claim a threat to international peace and security that would normally require Security Council action? If international law is to offer meaningful criteria for and constraints on the resort to war, the recent tendency to merge different justifications for the use of force into one category of threat prevention must be resisted.