This thesis starts with an attempt to build cosmopolitan morality into war crimes law. Universalist, individualist and rationalist cosmopolitan principles can provide moral meaning to the law. A cosmopolitan war crimes law embodies a cohesive and concerted cosmopolitan community. Individuals can be bound by universal moral links or certain models of political organisation such as cosmopolitan constitutionalism or cosmopolitan democracy. The community values underlying such a law are universal and homocentric. Social foundations of the cosmopolitan law is linked to certain assumptions of the moral nature of human beings. The cosmopolitan community relies on its rational and autonomous members to achieve consensus on fundamental values and account for war crimes. On the ideational level, cosmopolitan values are inevitably curtailed in the criminalisation process for the law to have any relevance in war. On the experiential level, cosmopolitan legal legitimacy is diluted by the distance between remote strangers, and diminished by the countervailing social environment. The description of effects of war on human institution and agency turns cosmopolitanism against its own premise – the constancy and impartiality of the moral nature of human beings. The confrontation of cosmopolitanism and war is not purely a contestation of wills. The thesis shows that potential challenges of a cosmopolitan war crimes law lie in the subconscious and unconscious mind, not just conscious thinking; in radical emotions, not just rational communications. The moral distance between the embedded person and a cosmopolitan community can undermine a cosmopolitan war crimes law’s effectiveness and relevancy. This thesis suggests a change of perspective: a cosmopolitan war crimes law’s normativity should be seen as relative and conditional, not comprehensive and absolute. It also suggests incorporation of empirical insights into legal reasoning and creation of conducive social and normative environments for a cosmopolitan war crimes law.