This chapter compares the social systems and intergroup relations in chimpanzees and bonobos – our two closest extant relatives – to illuminate the evolutionary roots of human intergroup lethal conflict and peaceful intergroup cooperation. Notably, among chimpanzees, intergroup encounters are typically hostile and sometimes lethal. Bonobos, by contrast, exhibit greater variability in their intergroup behaviour. They do not carry out intergroup coalitionary killings and occasionally engage in some forms of intergroup cooperation, although these are largely confined to one-on-one (dyadic) interactions. By comparing the social systems and intergroup relations of these species, the chapter can help shed light on the roots of the human inclinations for both intergroup lethal conflict and peaceful cooperation.